A detailed physical map of the porcine MHC class III region on Chr 7 was constructed with a panel of probes in a series of hybridizations on genomic pulsed field gel electrophoresis (PFGE) Southern blots. A precise organization of the 700-kb segment of DNA between G18 and BAT1 can now be proposed, with more than 30 genes mapped to it. Comparison of this region with homologous regions in human and mouse showed only minor differences. The biggest difference was observed in the CYP21/C4 locus with only one CYP21 gene and one C4 gene found, whereas in human and mouse these genes are duplicated. These results show the class III region is very well conserved between pig, human, and mouse, in contrast with the class I and class II regions, which seem more prone to rearrangements.