From 1962 to 1969 the Anaerobic Bacteriology Laboratory received 114 isolates of nonhistotoxic Clostridia from human blood specimens. We studied the clinical and bacteriologic data for 86 patients with bacteremia caused by 12 groups or species of these Clostridia. Eighteen patients were newborns. The bacteremia was commonly associated with intraabdominal lesions, hematologic disorders, genitourinary diseases, recent surgical procedures, cardiac diseases, intercurrent antibiotic therapy, and tumors or leukemias. Almost all of the patients had fever and leukocytosis; some also had hypotension. Clinically, their condition resembled Gram-negative bacillus septicemia, sepsis neonatorum, or fever of unknown origin. The clinical course was typically self-limited and was probably not affected by antibiotic therapy. An endogenous origin of the Clostridia was proposed, since only two patients had external trauma and none had gas gangrene. As with so many other opportunistic organisms, these nonhistotoxic Clostridia may invade systemically when host defense mechanisms are lowered by underlying illnesses.