A large difference in the number of gamma-IFN receptors was found on a variety of human tumor cell lines with a range of 0.4 to 15 X 10(3) binding sites/cell. The receptor number did not correlate with the potential responsiveness of the cells to gamma-IFN, in regard to either growth inhibition or induction of HLA-DR expression, two independently regulated gamma-IFN effects. However, with respect to HLA-DR expression it was found that the gamma-IFN sensitivity of inducible cell lines depended on the number of receptors, so that with increasing number of receptors present on these cells lower doses of gamma-IFN were required for induction of the gamma-IFN effect.