alpha1-Antitrypsin types were determined in 200 individual specimens of Malaysian Macaca irus. We found the pattern B in 76 samples, BC in 116, and C in 8. Assuming that these patterns are determined by codominant alleles at one locus, this distribution constitutes a significant (P less than 0.001) deviation from Hardy-Weinberg equilibrium, because of an excess of BC and low prevalence of C. We found no clear evidence for the presence of alpha1-antitrypsin deficiency comparable to the situation in man.