Rhodococcus equi, a gram-positive, weakly acid-fast coccobacillus, initially isolated from horses, is becoming increasingly recognized as an important pathogen for immunosuppressed human hosts since the first human case was reported in 1967. A review of the English medical literature yielded 53 cases. During the last 11 years, the microbiology laboratories of the authors isolated the organism from 12 patients. Of the total 65 cases, 60 occurred in immunosuppressed patients with HIV infection, malignant neoplasms, or chronic immunosuppressive therapy. The lung is the most common primary site of infection. Typically, the lesion is densely infiltrated by histiocytes with multiple microabscesses. Intracellular gram-positive coccobacilli are easily demonstrated. R equi grows well on routine non-selective media at 35 degrees C. Previously, many cases may have been missed because the organism resembles oropharyngeal commensal diphtheroids. Clinical information with gram and Kinyoun strains on fresh isolates is helpful in recognizing the possibility of R equi infection.