Friend erythroleukemia cells were induced by dimethyl sulfoxide (DMSO) into erythroid differentiation, as characterized by the production of hemoglobin. Induction increased with DMSO concentrations up to 1.5% v/v, at which point about 90% of the cell population produced hemoglobin as measured by a benzidine-staining technique. Heat treatment at 39.0-40.5 degrees C during a 7-day-incubation period, for differentiation in the presence of DMSO, resulted in the inhibition of hemoglobin induction. Also, acute heat treatments at 41.5-46.0 degrees C before or after the addition of DMSO resulted in the inhibition of DMSO induction. This effect was greatest when DMSO was present during heating. The results support the conclusion that hyperthermia inhibits the differentiation process which is induced by DMSO treatment.