Human serum beta 2-glycoprotein I was analyzed by isoelectric focusing followed by immunoprinting or immunoblotting with monospecific antiserum. Isoelectric focusing revealed a heterogeneous pattern consisting of 4 major and 4-5 minor bands with isoelectric points of the major bands between pH 5.4 and 6.2. Comparative analysis of sera from more than 400 healthy blood donors showed individual variations of band patterns: six different phenotypes were observed. A family study of 44 families with a total of 129 children demonstrated the genetic control of this variation. Presumably, three alleles, called B2G*1, B2G*2, and B2G*3, determine six phenotypes: B2G 1, 2, 3, 1-2, 1-3 and 2-3. The phenotype 3, however, has not been found in this study. An additional phenotype, noted in one serum specimen, was tentatively classified as B2G 2-4. The distribution of phenotypes and alleles in two populations, from Munich and from Tyrol, has been examined and the frequencies are presented. This genetic polymorphism appears not to be associated with inherited quantitative variations of beta 2-glycoprotein I found earlier. The inherited variations can still be recognized after treatment of sera with neuraminidase and with endoglycosidase F, although the banding pattern is altered and shifted towards the cathode. The genetic polymorphism can, therefore, not be ascribed to variations residing in the carbohydrate side chains.