We have determined the incidence and clinical significance of positive microbiologic cultures in a series of 290 peripheral blood stem cell concentrates in 95 patients undergoing multiple apheresis procedures for autologous stem cell rescue. Specimens for bacterial cultures were obtained after processing of the autografts just prior to freezing. The incidence of microbial contamination was 4.5% (n = 13). The predominant pathogenic microorganism cultured was coagulase-negative Staphylococcus (n = 11). From 8 patients with contaminated leukapheresis products 6 underwent autologous stem cell transplantation. Five patients received 1-5 culture-positive stem cell concentrates without serious sequelae, whereas the sixth patient was autografted with noncontaminated leukapheresis products, 1 concentrate contaminated with Aspergillus fumigatus being not reinfused. No microorganism present in the stem cell autograft was recovered in vivo in the posttransplantation period, although fever as a sign of infection occurred in all but 1 patient. Peripheral blood stem cell collection and ex vivo processing for cryopreservation may result in microbiologic contamination. However, our data show that infusion of contaminated stem cell autografts does not play a significant role as a source for infections in the clinical setting of autologous stem cell rescue.