A bloodstain found on a jacket belonging to one of the men accused of murdering Stephen Lawrence was caused when the blood was fresh, the Old Bailey heard today.
The tiny stain, measuring 0.5mm by 0.25mm, was discovered on the collar of a jacket belonging to Gary Dobson by a cold case team.
Defence counsel argue that it was caused when an old, dried blood flake got on to the jacket via contamination and was dissolved during tests for saliva.
But today forensic scientist Edward Jarman told a jury at the Old Bailey that his experiments suggested the stain was made by fresh blood.
He said that old fragments of Mr Lawrence's blood became "gel-like" during saliva testing and therefore would not soak into fabric.
Mr Jarman explained: "When these gel fragments were left to dry they became adhered to the fabric so when you touched them they stuck securely to it, but they had not absorbed into the fabric."
When he repeated the test using flakes made from fresh blood, they dissolved and caused a stain.
Dobson, 36, and David Norris, 35, are accused of being involved in the gang attack that killed Mr Lawrence in Eltham, south east London, in April 1993, which they deny.
Mr Jarman was part of a team at a company called LGC asked to help in a cold case review of Mr Lawrence's murder.
Earlier, the jury was told that he made key findings when he used sticky tape to gather debris from the original police evidence bag used to hold Dobson's jacket.
Three fragments of blood were discovered, which had a chance of less than one in a billion of not being Mr Lawrence's blood.
He also found a fragment of blood that encased three textile fibres, which, he told the court, meant the blood was wet when it came into contact with them.
Scientists could not get a full profile from this for DNA testing, but from a partial profile found it had a probability of less than one in a billion of not belonging to Mr Lawrence.
Mr Jarman tested flakes both from Mr Lawrence's clothing and from the evidence bag.
When they became "gel-like" after saliva testing they did not give out any fluid or material that could have caused staining, he said.
In a conclusion after his testing, he said: "I've seen nothing to suggest that the wetting of the item during Phadebas (saliva) testing resulted in loose fragments of blood producing the stain on the collar."
But he admitted this could not be entirely excluded as there may have been undiscovered flakes that behaved differently.
Mr Jarman said variation in the colour of the blood stain could suggest the blood was partially dried at the time the mark was caused, but it was so small it was hard to draw conclusions.
He explained that the LGC team worked on the premise that only a small amount of blood would have been available to be transferred on to clothing during the attack.
Drops could have flown through the air, or been passed on if the knife touched someone's clothing, he told the court.
Mr Jarman said a stain of the size found on Dobson's jacket would only have taken "a couple of minutes" to dry.
If the blood had been passed on to the jacket indirectly, he said: "The transfer would have had to have occurred very shortly after the incident given the sort of quantities of blood that are likely to have been available during the assault."
Mr Jarman said that any blood transfer process that involved multiple steps was unlikely.
He said: "The collar stain was more likely to be the result of primary route transfer than other more complex routes."
Blood flakes found in the jacket's evidence bag could have dried and broken off the stain on the collar, the jury was told, or other stains which had been on the jacket but left no trace.
Testing for blood carried out on a cardigan seized from Dobson's house, and jeans, a sweatshirt and another cardigan taken from Norris's home were inconclusive.
The trial was adjourned until tomorrow.