A live toad the size of a fist popped out on to her kitchen table, leaving the 42-year-old teacher from Northumberland screaming so hard her chest hurt.
Mrs Henderson, of New Hartley, had bought the pre-packed salad at a Sainsbury's store in Whitley Bay.
Managers for the supermarket chain are trying to find out how the toad got into the salad which is shipped in from Africa and packed in Britain.
Mrs Henderson, who pulled the toad out of the bag by its leg as she nibbled a piece of lettuce, could need medical tests to make sure she has not caught any disease from it.
Staff from Sainsbury's collected the errant animal from her home and it is now being examined by experts to establish its origins.
Mrs Henderson said: "I was so shocked and frightened I backed against the wall and was screaming the place down. I was screaming that much I was having chest pains. It was alive when I found it so it must be very resilient.
"The staff from Sainsbury's were as shocked as I was when they saw how big it was.
"It was the size of a fist and they couldn't understand how it got there." A Sainsbury's spokesman said: "The toad, if that is what it is, is alive, and is being checked to see where it is from. We won't have to kill the creature to carry out the tests.
"This case is a priority so we hope to get the results very soon.
"If it has come from abroad we will obviously advise Mrs Henderson what she should do next.
"We have launched an investigation and will be in touch with the supplier to see how this could have happened.
"We have screening procedures which should have spotted something like this so we'll have to look at why they didn't pick it up."Reuse content