Summary.Genetic variants of pig serum alpha‐protease inhibitors (protease inhibitors‐1 and ‐2, PI1 and PI2; postalbumin‐1A and ‐1B, PO1A and PO1B) were studied by 2‐D electrophoresis of serum samples. The inheritance data confirmed the close linkage between the loci of these inhibitors. The order between these loci was indicated asPi1‐Po1A‐Po1B‐Pi2and these were spread over a distance of about 1cM. Very strong linkage disequilibrium was observed between the alleles at these loci. The two breeds studied (Belgian Landrace and Piétrain) showed very different allele and haplotype frequencies. Both breeds showed extensive polymorphis