To identify genes mediating the antiproliferative action of interferon (IFN), two cDNA libraries were constructed with mRNA from IFN-treated and untreated human fibrosarcoma (HT1080) cells previously shown to be highly sensitive to the antiproliferative effects of IFN. Differential screening of these two libraries identified cloned sequences whose expression was either induced or repressed with IFN treatment. Rescreening of these sequences with cDNA probes constructed from proliferating or quiescent cells led to the identification of one IFN-induced and three IFN-repressed sequences whose expressions also appeared to be modulated by cell proliferation. Blot-hybridization analysis revealed that RNA levels corresponding to the three repressed genes decreased when HT1080 cells were treated with IFN or when proliferation of normal CUA foreskin fibroblast cells became naturally arrested by contact inhibition. Levels of RNA corresponding to the induced gene increased in HT1080 cells within 24 hr after IFN-treatment but declined below basal levels by 48 hr. Expression of these genes was unaffected or only slightly affected by IFN treatment in variant cells resistant to the antiproliferative effects of IFN. Collectively, these results suggest that the identified cDNAs correspond to genes that are involved in the antiproliferative action of IFN. Moreover, these results also suggest that IFN's antiproliferative action may be exerted through genes that contribute to arresting cell proliferation during contact inhibition.