Friend erythroleukemia cells were induced to differentiate in a chemically defined medium in which bovine serum albumin was substituted for fetal calf serum. The inducers used to induce differentiation fell into two groups. Group A included inducers such as hexamethylene bisacetamide and dimethyl sulfoxide that could not stimulate differentiation and were very toxic to the cells over a wide range of concentrations. However, addition of L-ornithine but not D-ornithine resulted in a substantial degree of differentiation approaching the degree of differentiation reached by these inducers in the presence of fetal calf serum. A second group of inducers included short-chain fatty acids, actinomycin D, and the aminonucleoside of puromycin which did induce differentiation in the absence of ornithine to a degree comparable to that reached by the same inducers in the presence of fetal calf serum. These results support our previous finding that polyamines are involved in the triggering of differentiation and the classification on this basis of the currently used inducers.