Amodiaquine is used for antimalarial prophylaxis and treatment and has been associated with neutropenia and agranulocytosis in man. The effect of the drug on the in vitro growth of bone marrow human myeloid progenitor cells (GM-CFU) was tested using the soft agar culture technique: 42 haematologically normal subjects were studied and it was found that amodiaquine, at concentrations tested in vitro (0.005, 0.05 and 0.5 micrograms.ml-1), had no quantitative effect on the colony and cluster growth. Our results argue against direct toxicity of the drug on GM-CFU. Therefore, in cases of amodiaquine-associated neutropenia, alternative mechanisms should be considered: a abnormally sensitive GM-CFU; b) toxic effect of metabolites such as desethyl-amodiaquine; c) immune-mediated toxicity.