We determined the prevalence and clinical significance of positive microbiologic cultures obtained from 194 consecutive bone marrow harvests intended for infusion in 188 consecutive adult bone marrow transplant patients. Microbiologic cultures were obtained at harvest, after manipulation in vitro (for ABO imcompatibility or purging procedures), and at the time of thawing and infusion (after earlier cryopreservation). Only one of 194 marrow harvests was culture-positive intra-operatively (from an ABO-compatible allogeneic marrow that was infused without manipulation). None of 39 other allogeneic marrows (including 21 ABO-incompatible marrows which were manipulated in vitro) and none of 154 autologous marrows (including 40 which were purged in vitro) grew bacteria or fungi. On the other hand, 12 of 153 (8%) bone marrow samples were positive for micro-organisms after thawing at the time of infusion. The predominant organisms cultured were gram negative bacilli (including five Pseudomonas sp.), probably introduced during the thawing process in a water bath. In only one of 13 contaminated marrows was the same organism(s) (Pseudomonas picketti and Pseudomonas paucimobilis) recovered in vivo during the transplant course. This patient experienced a bacteremia which was eradicated without sequelae. Contamination of marrow can occur during the procurement and in vitro handling processes. With proper sterile technique bone marrow infusion does not pose a significant infectious risk for the immunocompromised transplant patient.