Given the prominent position of pig endothelial cells in pig-to-human xenotransplantation and the role of classical and nonclassical MHC class I proteins in T and NK cell recognition, the expression of pig MHC (SLA) class I genes in a pig aortic endothelial cell line (AOC cells) was examined. Using a primer corresponding to a highly conserved region of exon 4, RT-PCR analysis of SLA class I expression in AOC cells revealed not only expression of the classical SLA class I ( SLA-1, -2, and -3) genes, but also SLA class I transcripts corresponding to SLA nonclassical class I (class Ib) genes SLA-6 and SLA-8. Further analysis of SLA class Ib expression in porcine aortic endothelial cells using SLA class I gene-specific primers confirmed SLA-6 and SLA-8 expression and also demonstrated expression of SLA-7. While SLA-6 has been relatively well characterized, no data regarding bona fide SLA-7 and SLA-8 transcripts have been reported. Therefore, cDNAs containing the complete open reading frames of SLA-6, -7, and -8 were obtained. Compared to an SLA-1 protein sequence, the predicted SLA-7 and -8 protein sequences exhibited most sequence divergence in alpha1, alpha2, and cytoplasmic domains. Expression of SLA-6, -7, and -8 was examined by RT-PCR using RNA prepared from a variety of tissues. SLA-6 transcripts were detected in every tissue examined. Except for brain, SLA-8 transcripts were similarly widespread. SLA-7 exhibited more limited tissue distribution.