We recently established a human granulocyte‐macrophage colony‐stimulating factor (GM‐CSF)‐dependent cell line (HML) from colony‐constituent cells grown by peripheral blood cells of a patient with acute megakaryoblastic leukaemia. The HML cells possessed megakaryocytic features, as determined by cytochemical, electron microscopic and flow cytometric analysis. In the present study we examined the effects of retinoic acid (RA) on the development of HML cells. All‐trans‐RA, 13‐cis‐RA and 9‐cis‐RA at 10⁻⁸ mol/l to 10⁻⁵ mol/l inhibited the GM‐CSF‐dependent cell growth. Some of the RA‐treated cells contained prominent azurophilic granules and were positive for peroxidase. They also reacted with Biebrich scarlet, Luxol fast blue and a monoclonal antibody against eosinophil peroxidase. In addition, exposure to RA increased the frequency and the intensity of major basic protein‐positive cells. However, eosinophil‐derived neurotoxin and eosinophil cationic protein were not detected or were only detected at a low level in the lysates of the HML cells treated with RA. Although IL‐5 alone could not stimulate cell growth, the addition of IL‐5 to the cultures containing stem cell factor + all‐trans‐RA was required for the expression of the eosinophilic phenotype. These results suggest that the HML cell line is a megakaryoblastic cell line with the potential to differentiate into the eosinophilic lineage. HML cells may be a useful model for elucidating the eosinophilic differentiation programme.