The human parasite Trypanosoma brucei gambiense grew continuously at 37 degrees C in primary cultures of murine bone marrow. Cultured parasites remained virulent for mice. Rapid parasite growth coincided with the appearance of adherent adipocyte-epitheloid cell aggregates that also promoted hematopoiesis. This culture system should permit studies of host cell control of trypanosome proliferation, pathogenic effects of trypanosomes on blood cell development, and the relative trypanocidal and marrow suppressive activities of drugs.