A segment of 158,063 nucleotides of the pig major histocompatibility complex (SLA) and corresponding to the junction of the class I and class III regions was sequenced entirely. The centromeric part of the segment contained six class III genes including the three tumor necrosis factor genes, while the telomeric part contained three genes belonging to the class I region. The order and the molecular organization of these genes were exactly conserved in the SLA and HLA complexes, except for the SC1 gene which displayed a shift of the reading frame in swine. The cluster of the three SLA class I-related genes (Ib) and the MIC1 and MIC2 genes were located in the middle of the segment, in the following order from the centromeric side onwards, SLA-6, SLA-7, SLA-8, MIC-1 and MIC-2. All three SLA Ib genes displayed an overall molecular structure compatible with the expression of membrane-anchored glycoproteins. The SLA-7 and SLA-8 genes bear greater resemblance than to the SLA-6 gene. Six SLA-6 alleles have been previously defined differing each from the other by unique point mutations. One of them, appeared to have arisen through the occurrence of a gene conversion event in which the SLA-7 gene served as template. Only MIC-2 gene might be functional, the second MIC-1 gene being truncated. In all, the 14 genes characterized spans 37% of the total sequence. The remaining 63% nucleotides comprised a number of repeat DNA motives, including LINE fragments, SINEs, microsatellites, and also numerous nucleotide stretches not yet defined in swine.