Six clinically normal ponies were infected intravenously with equine infectious anaemia virus (EIAV) to determine the effect of EIAV on numbers of erythroid, granulocyte/monocyte and fibroblastic progenitors in the bone marrow. Bone marrow progenitor assays were performed at weeks 1, 2, 3, 4 and 8 postinfection. The late erythroid progenitors, colony-forming units-erythroid (CFU-E), were suppressed at each time point postinfection with the maximal suppression occurring at week 2 postinfection. The maximal suppression corresponded to the peak of plasma virus concentration. The maximal suppression of the early erythroid progenitors, burst-forming units-erythroid (BFU-E), also occurred at week 2 postinfection. Removal of adherent cells from the bone marrow mononuclear cells (BMMC) abrogated the suppression. Neither granulocyte/monocyte progenitors (CFU-GM) nor fibroblastic progenitors (CFU-F) were affected by EIAV infection.