Flow cytometry analysis was applied to swine chromosomes prepared from phytohemagglutinin (PHA) stimulated peripheral blood lymphocytes. Flow karyotypes from both sexes and from t(3;7) translocation carrier females were obtained. A certain number of chromosome pairs could be assigned to various peaks. In fact, 13 peaks were observed for 18 autosomal pairs plus X and Y. Moreover, abnormalities owing to the t(3;7) translocation were readily observable. The number of base pairs for chromosomes associated with the various peaks was estimated by comparison with human flow karyotypes. The following four peaks were thus sorted: the peak assumed to represent the translocated chromosome 7 plus the normals associated with it; the corresponding peak from a normal swine; the peak assumed to contain among others the normal chromosome 7; and finally the peak corresponding to swine chromosome 1. Chromosomes of each peak were collected on Pall Biodyne membrane. Following appropriate denaturation and prehybridization, the four samples were hybridized with a human leucocyte antigen (HLA) class I 32P-labelled cDNA probe, representing most of the coding sequence of the HLA B7 gene. The results confirmed previous data from other techniques that assigned the swine MHC(SLA) to chromosome 7. Subsequently, sorted samples were hybridized with a porcine genomic Interferon alpha probe in order to confirm the mapping of this gene family on porcine chromosome 1.