Cells of two human neuroblastoma lines, GOTO and KP-N-RT-LN, differentiated in response to treatment with 1,000 IU/ml of recombinant human interferon-gamma (rIFN-gamma), but did not respond to rIFN-alpha 2 or IFN-beta. Treatment with rIFN-gamma rapidly increased the phosphorylation of several cell proteins; in particular, there was enhanced phosphorylation of a 61-kD protein within 1 min after treatment. This enhancement was not observed in variant sublines of GOTO cells, which did not differentiate when treated with rIFN-gamma. These findings suggest that when neuroblastoma cells are induced to differentiate by treatment with rIFN-gamma, phosphorylation of the 61-kD protein plays an important role.