Expression of the Ly-6A/E gene by transformed cells was investigated in 14 cell cultures of C57BL/6 and BALB/c mouse origin derived from spontaneous or chemically-induced non-hematopoietic tumours and from cells transformed by SV40 virus. Histologic types included carcinomas, sarcomas and a melanoma. Thirteen out of 14 cell cultures expressed membrane Ly-6A/E antigens; only the B16-A melanoma was negative, but it expressed Ly-6A/E after incubation with IFN-gamma. The effect of in vitro permanence was studied on early (< 10) and late (> 20) passages of B16-A melanoma and MN/MCA1 fibrosarcoma. Late passage B16-A cells were Ly-6A/E-negative and refractory to induction of Ly-6A/E (but not of H-2 antigens) by IFN-alpha, IFN-beta, or IFN-gamma; MN/MCA1 maintained a high expression of Ly-6A/E, but no increase was induced by IFNs. It was found that both early and late in vitro passages of MN/MCA1 actively produced IFN-alpha/beta. The analysis of cells of fibroblastic origin revealed a significant correlation between IFN release in the culture medium and Ly-6A/E levels. Culture of fibrosarcoma cells in the presence of an anti-IFN-alpha/beta serum reduced Ly-6A/E expression, thus indicating the existence of an autocrine loop.