The alpha 1-antitrypsin (AAT) or protease inhibitor (Pi) genetic polymorphism was studied in 144 white, 100 coloured, 104 Indian and 127 black (Northern Sotho) healthy individuals (controls), in the Pretoria area. Their Pi phenotype and gene frequency distributions are compared with world-wide data on other population groups. The severely deficient Pi phenotypes S, Z and SZ jointly attain frequencies of 0.3-0.4% in coloureds and whites; in blacks and Indians the corresponding frequencies are very much lower. The implication for preventive medicine and public health is that in South Africa the sequelae of Pi deficiencies such as cirrhosis of the liver and/or emphysema of the lung are of practical importance in whites and coloureds and much less so in blacks and Indians. In 176 white breast cancer patients studied, the Pi phenotype and gene frequency distributions were found to be similar to those of healthy controls (not statistically significant). Cohorts of other patients were also phenotyped because of their low alpha 1-globulin concentrations in routine serum protein electrophoresis and/or their specific disease condition (cirrhosis of the liver or emphysema of the lung) known to be associated with AAT deficiency. These results are discussed in terms of their significance for family follow-up, genetic counselling and a preventive service. The need to avoid atmospheric pollution, especially cigarette smoke, is emphasised as a major and cost-effective preventive measure.