An in vitro examination was made of the synergistic antiproliferative effect of recombinant interferon-gamma (IFN-gamma) with interferon-alpha 2a (IFN-alpha 2a) on human tumor cell lines, including lung small cell carcinoma QG-90, amelanotic melanoma HMV-1, renal carcinoma ACHN and Burkitt lymphoma Daudi. Sixty-four percent of HMV-1 cells and 75% of ACHN cells seeded were killed by treatment with 10 pM and 1000 pM of IFN-gamma, respectively. Furthermore, 69% of HMV-1 cells and 59% of ACHN cells were killed by 1000 pM of IFN-alpha 2a. When HMV-1 was treated first with 10 pM of IFN-gamma, and then with 1000 pM of IFN-alpha 2a, 81% of cells seeded were killed (p less than 0.001). Similarly, 89% of ACHN cells were killed by the same sequential treatment with 1000 pM IFN-gamma, and then of IFN-alpha 2a (p less than 0.001). However, adverse sequence of treatment could not produce such a synergistic result. On the surface of these susceptible cells, HMV-1 and ACHN, to both IFNs, the number of receptors for IFN-alpha 2a increased significantly after treatment with IFN-gamma without any serious change of the dissociation constant, suggesting that increase in the number of receptors for IFN-alpha 2a may be the major mechanism of the synergistic effects of IFN-gamma with IFN-alpha 2a.