Using immunoselection, we have isolated 11 independent mutant HT1080 fibrosarcoma cell lines defective in the induction by interferon gamma (IFN-gamma) of the expression of the human leukocyte antigen HLA-DRA. The mutations are recessive and fall into five complementation groups. All the mutants are affected mainly in the expression of major histocompatibility complex class II and invariant-chain genes. Type I mutants (three complementation groups) are completely defective in induction of the invariant-chain and class II HLA-DP, -DQ, -DR, and -DM genes, whereas type II mutants (two complementation groups) induce these genes weakly in response to IFN-gamma, in the order DPB > DRA > invariant chain. The induction by IFN-gamma of the mRNAs for class I, TAP1, LMP7, and 9-27 is partially defective and the induction of the proteins IRF-1 and ICAM-1 is normal in both types of mutants. All the mutants respond normally to IFN-alpha. The mutants are stable and thus can be used to clone the affected genes by reversion.