Thymocytes and leukemia cells of some mouse strains yield TL proteins, precipitable by anti-TL antiserum and by anti-TL monoclonal antibodies, that include not only the familiar heavy (H) chain of 45-50 kDa but also products of higher molecular mass. Production of a 53-kDa TL form by Tlad thymocytes was studied in detail. A cross was made between B10.M (Tlad) mice, which produce the 53-kDa TL, and mice of the A strain (Tlaa), which make only the usual H chain. Hemi-expression of apparently unaltered 53-kDa TL was observed in thymocytes of the Tlad/Tlaa heterozygous F1 progeny. Thus, there was no indication of positive or negative trans interaction with respect to production of the 53-kDa TL form associated with Tlad. We conclude that production of 53-kDa TL is governed intrachromosomally. Two-dimensional chymotryptic peptide maps of the TL H chain and the 53-kDa TL of Tlad thymocytes differed only by added features found in the map of the 53-kDa TL. With the exception of Tlaa, all Tla alleles (Tlab-f) yielded TL products of higher molecular weight than the accompanying H chain, although in the case of Tlab this was evident only in TL+ leukemia cells because Tlab thymocytes are TL-. For H-2, representing other class I genes, no products other than the familiar H chain were demonstrable under similar conditions.