The major histocompatibility complex (MHC) of the BALB/c mouse contains three genes encoding classical class I molecules, as well as at least 32 nonclassical class I genes. Although much is known about the genes encoding the classical class I molecules, the majority of the nonclassical genes have not been characterized. This report describes a newly identified nonclassical class I gene, Thy19.4, which contains an open reading frame and resembles in several regards the genes encoding classical class I molecules. The similarities include shared amino acid sequence motifs which suggest that the putative Thy19.4 molecule may assume a tertiary structure similar to that of the classical class I molecules, as well as widespread transcription in a variety of tissues. However, unlike the classical class I genes, the Thy19.4 gene maps approximately 1 cm distal to the Tla region of the MHC, in the same region as the gene encoding the Hmt element of the maternally transmitted antigen.