Green and Kehinde [(1974) cell 1, 113-116] have isolated clones of Swiss 3T3 fibriblasts that are able to convert to adipose cells. In this paper we report on two chemicals (prostaglandin F2alpha, 0.1 mug ml, and 1-methyl-3-isobutyl xanthine, 0.5 mM) that are able to rapidly and irreversibly program the fibroblasts to differentiate into adipose cells. Confluent cultures treated with prostaglandin F2alpha and insulin for 3-5 days, followed by insulin alone for 7-48 hr, yield numerous adipocyte colonies compared to control dishes and dishes treated with insulin alone. Cells treated with prostaglandin F2alpha or 1-methyl-3-isobutyl xanthine alone, rinsed, and then exposed to insulin gave similar results, indicating that the continuous presence of the triggering agent is not required to elicit the conversion of the fibroblasts to adipocytes. Agents that raise intracellular levels of 3':5'-cyclic AMP. 1.0 mM; 8-bromo-cyclic AMP,0.5 mM; and prostaglandin E1, 0.1 mug/ml) do not trigger the conversion process, suggesting that cyclic AMP may not be the mediator of differentiation in these cells. 8-Bromo-cyclic AMP, however, does induce thase; 3':5'-nucleotidohydrolase; EC 3.1.4.17) in these cells; the induction appears to be mediated by increases in intracellular cyclic AMP levels. These results indicate that this cell line might offer a system for studying the regulation of a type of cellular differentiation.