A needleless syringe was thrown at Barry Bonds during San Francisco Giants' 6-1 season-opening Major League Baseball defeat at San Diego Padres.
Bonds endured a hostile reception following doping allegations
Bonds was booed throughout the game, as fans vented their fury over claims that he had taken illegal steroids.
The 41-year-old, who has denied the allegations, aims to break the all-time career home run record this year.
He said: "That's the first time someone threw something like that. I put it off the field so no-one would get hurt."
A programme of 13 games took place on Monday, following Chicago White Sox's opening win on Sunday.
The World Series winners began their quest for a second successive Major League Baseball championship in style.
White Sox dominated the rain-hit season opener, cruising to a 10-4 victory over the Cleveland Indians.
Bonds, who missed most of last season with a knee injury, began the season with 708 career home runs, six shy of the legendary Babe Ruth and 47 behind Hank Aaron's all-time best.
But his last few seasons have been dogged by allegations of doping and he has been linked with the disgraced San Francisco laboratory Balco.
MLB commissioner Bud Selig says plans are in place to commemorate Bonds passing Ruth's and Aaron's records.
But he has also announced an independent inquiry into doping in the game, including the allegations against Bonds.