Work from our laboratory indicates that HLA class II induction by IFN- gamma in the retinoblastoma (RB) protein-defective breast carcinoma line MDA-468-S4 (S4) requires reconstitution of functional RB. To determine whether RB is required for HLA class 11 expression in multiple tumor types, the RB-defective non-small cell lung carcinoma line H2009 and its RB-reconstituted subclones were examined for class II inducibility. Surface HLA-DR (DR) was not inducible by IFN-gamma in H2009. However, unlike the RB-reconstituted subclones of S4, DR surface expression was not detected in the H2009 RB-positive subclones. IFN-gamma induction of CIITA, a major regulator of class II transcription, suggested that H2009 retained at least part of the IFN-gamma signaling pathway leading to class II expression. Examination of class II mRNA indicated that IFN-gamma induction of RB was rescued in the RB-positive subclones of H2009, confirming the requirement for RB for HLA class II inducibility and revealing that RB is required for inducibility in developmentally distinct tumor types. However, DRA inducibility was not rescued in the H2009 RB-positive subclones, which explained the lack of surface DR induction in the RB-positive H2009 subclones. DPA and DPB were also only weakly inducible in the RB-reconstituted H2009 subclones, compared with the previously described, S4 RB-positive subclones. Finally, data reported here indicates that RB's ability to inhibit IFN-gamma-induced apoptosis is not a viable explanation for why RB expression rescues DRB inducibility in H2009.