The ability of human marrow erythroid progenitors growth in diffusion chamber culture has been assessed. Chambers containing cells (2 X 10(5)) suspended in clot were implanted into peritoneal cavity of anemic rats. The anaemia was induced by phenylohydrazine (50 mg/kg body weigh). Erythroid colonies were counted on day 1, 2, 3, 5, 7, 9, 11, 13 and 15. Two types of colonies formed by erythroid progenitors (BFU-D-E, CFU-D-E) were determined using criteria of colony size and culture duration. The count of colonies formed by CFU-D-E was highest on day 2 (29 +/- 4) and underwent reduction in subsequent days. Colonies formed by BFU-D-E were seen beginning of day 5 and their number peaked on day 7 and 9. These results indicate that the best time for determining the growth of human marrow CFU-D-E and BFU-D-E in diffusion chamber culture is on day 2 and day 7-9, respectively.