Expression of the human class I MHC loci, HLA-A, -B, and -C, was examined by reverse transcription and competitive PCR with locus-specific primers. This approach allows unambiguous identification of target sequences by analysis of the amplified products. JY and Pala lymphoblastoid B cells express more HLA-A than HLA-B mRNAs and little HLA-C mRNA. Raji Burkitt lymphoma and HeLa carcinoma cells express approximately equal amounts of HLA-A and HLA-C mRNAs but less HLA-B mRNA. Jar trophoblast cells express no HLA class I mRNAs. Surprisingly, K562 leukemia cells express significant amounts of HLA-C mRNA. However, K562 cells contain no detectable HLA-A or -B mRNAs, suggesting that these loci are regulated independently. Furthermore, cultured endothelial cells and smooth muscle cells express low, approximately equal amounts of HLA-A, -B, and -C mRNAs, whereas donor-matched, EBV transformed B cells express much more HLA-B mRNA, suggesting that cell type dependent regulation underlies differential locus expression. Finally, expression of HLA class I molecules on the cell surface correlates with total HLA mRNAs but not with mRNAs encoded by any one locus. Differential expression of these HLA class I loci may contribute to cell-type dependent immune reactions by preferentially presenting distinct peptides to T cells.