Class‐I‐MHC molecules are divided into class‐la molecules, which play major roles in recognition of virus‐infected cells, graft rejection and immune responses against tumors, and class‐lb molecules, which are less polymorphic and may be responsible for presenting unique classes of peptides. Our report characterizes RadLV‐induced thymic T‐cell lines that differ both in their tumorigenic potential and in the level of protein for class‐la and TL genes. The PDI.I cell line is CD4−CD8+ and expresses relatively high levels of class‐l as compared with the CD4+CD8+ PDI.2 cell line. These class‐I‐expression levels correlate with thymocytes and splenic T cells of the same phenotype, except that normal cells fail to express TL3b. Interferon‐treated PDI.2 cells demonstrate significantly lower levels of class‐l expression than do interferon‐treated PDI.I cells, and were shown to contain large amounts of degraded class‐l mRNA, at least some of which was TL in origin. These RNA products were not detected in PDI.I cells, suggesting the existence of a mechanism controlling cell‐specific and gene‐specific mRNA stability. Such RadLV‐induced cell lines provide a means for obtaining stage‐specific T cells, which can be used for studying the regulation of class‐l gene expression during T‐cell differentiation, as well as factors that differentially regulate class‐la and class‐lb expression and are potentially useful for studying T‐cell differentiation in general. © 1995 Wiley‐Liss, Inc.