Doohan's Honda had pole position but it was Biaggi, in his first season on 500cc bikes, who took the early lead at Mugello.
Doohan, four times the 500cc world champion, tailed Biaggi, threatening several times to pass him. On lap seven he made his move, sweeping through on the inside of a left-hand bend.
The Australian extended his lead in the closing laps to win comfortably, more than five seconds clear of Biaggi. Doohan's Spanish team-mate, Alex Criville, completed a clean sweep for Honda in third.
"My biggest problem was the warm-up this morning. The ground temperature was so low my tyres shredded," Doohan said. "The crew told me to take the first few laps easy to let the tyres warm up. But when Max set off so quickly I had to scrap that plan and try to chase him down. I figured that if I got close to him I'd go straight past and try and stay there."
Marcellino Lucchi won a rain interrupted 250cc race to claim his first victory at the age of 41, while Japan's Tomomi Manako won the 125cc class after Italy's Mirko Giansanti crashed while leading on the final lap.Reuse content