The effects of deltamethrin on bone marrow and spleen progenitor cell responsiveness to granulocyte and macrophage colony-stimulating factors were evaluated. Deltamethrin was tested in parallel with hydrocortisone to further investigate some similarity in the in vivo effects of both compounds observed in previous studies in our laboratory. In vivo effects were studied after the subcutaneous administration in mice of three 5 mg/kg injections of deltamethrin and a single 30 mg/kg injection of hydrocortisone to Balb/c mice. Soft agar colony formation, marrow and spleen cell counts, body, spleen and thymus weights were determined. Data obtained in vivo indicate that deltamethrin and hydrocortisone reversibly increase the formation of granulocyte and macrophage colonies in the marrow, but not in the spleen. No changes were observed in total and differential cell counts in the marrow and spleen and spleen weights. Treatment with both compounds, however, resulted in a dramatic reduction in thymus weights. Assays for endotoxin demonstrate that these effects were not due to the liberation of endotoxin. In vitro addition of 10(-5), 10(-6) and 10(-7) M deltamethrin and hydrocortisone to marrow cultures from untreated mice resulted in different effects from those observed in vivo. Hydrocortisone increased granulocyte and reduced macrophage colonies, whereas deltamethrin was without in vitro effects. It is suggested that deltamethrin effects are due to an indirect action on the hypothalamic-pituitary axis leading to increased corticosteroid levels. The importance of biotransformation mechanisms is also emphasized.