Marrow is cryopreserved for use in autologous bone marrow transplants, but little is known of the incidence of reactions in patients transfused with these cryopreserved marrows. Reactions in patients transfused during a 4-year period with 134 autologous marrows cryopreserved in dimethyl sulfoxide (DMSO) were compared with those in patients transfused with marrow that had been collected from HLA-compatible donors and that had not been cryopreserved. Patients transfused with cryopreserved marrow had significantly more nausea (44.8 vs. 14.1%; p less than 0.0005), vomiting (23.9 vs. 8.5%; p less than 0.01), chills (31.3 vs. 1.4%; p less than 0.0005), and fever (17.9 vs. 0%; p less than 0.005) than patients transfused with fresh allogeneic marrow. The incidence of emesis correlated with the dose of DMSO received, but that of nausea did not. All cryopreserved marrows were cultured for bacteria at the time of transfusion and 17 (12.7%) were found to be positive. Only 1 of the 17 patients transfused with culture-positive marrow developed sepsis during the transplant course with the same organism that was present in the transfused marrow. Although the reactions in donors transfused with cryopreserved marrow were readily treated, this study suggests that the incidence of some reactions might be decreased by reducing the dose of DMSO transfused. Bacterial contamination of transfused marrow was a worrisome complication, and efforts should be made to improve marrow collection and processing techniques to minimize that risk.