Seventy-seven patients (eighty-six lesions) who had been seen over a fifteen-month period because of an abscess at the site of injection due to parenteral abuse of drugs were identified in a retrospective review. Forty-one patients (forty-five abscesses) had had cultures before antibiotic therapy. Thirty (73 per cent) of the forty-one patients had isolation of a streptococcal species on culture, with microaerophilic streptococci identified in sixteen. Twenty (49 per cent) of the forty-one patients had isolation of a staphylococcal species. Four of the staphylococcal organisms were identified as oxacillin-resistant Staphylococcus aureus. Two patients who had three abscesses each had different organisms in each abscess. Gram-negative bacilli were identified in the cultures of ten (24 per cent) of the forty-one patients; patients who were forty years old or more had a sixfold greater risk of having gram-negative bacilli. Specimens of the abscess had been obtained from thirty-six patients for culture from twelve to seventy-two hours after the first dose of antibiotics had been given. The microbiological findings in these cultures were similar to those in the cultures of specimens obtained from patients before antibiotics had been given. Five (14 per cent) of thirty-five patients who had been tested for the human immunodeficiency virus had a positive result. This finding emphasizes the importance of surveillance for and precautions against the human immunodeficiency virus in people who abuse drugs parenterally.