One hundred samples of heroin and 100 of injection paraphernalia were cultured. A wide variety of microorganisms were found. The heroin contained Bacillus species, Staphylococcus albus, fungi, Clostridium perfringens, and (rarely) gramnegative bacilli. The same organisms were recovered from the injection paraphernalia, along with a variety of gram-negative bacilli among which Escherichia coli predominated. Not one coagulase-positive Staphylococcus was isolated, despite the fact that this organism is the most common cause of infections in our addict population; thus, the addicts themselves are probably the source of Staphylococcus aureus.