SummaryWhen rabbit serum labelled with vitamin B12[57Co] was subjected to starch gel electrophoresis and au;oradiography, three phenotypes of proteins capable of binding vitamin B12were observed. Family data revealed that these phenotypes (called TC‐A, TC‐AB and TC‐B) are controlied by two codominant alleles (TCAandTCB), at an autosomic locus.Proteins capable of binding vitamin B12both in vivo and in vitro are commonly referred to as Transcobalamins and can be found in the serum of numerous animal species (for a review, see Glass, 1974; Allen, 1975; Stenman, 1975). Furthermore, Daiger et al. (1975a) have described seven different patterns of vitamin B12binding proteins which occur in human plasma and which are presumably controlled by four alleles. The present paper describes experiments in which both starch gel electrophoresis and autoradiography are used to identify three phenotypes of rabbit serum proteins responsible for binding vitamin B12in vitro. It was found that these three phenotypes are controlled by two allelic codominant genes, at an autosomic locus.Individual serum samples (30 μl), obtained from 385 White New Zealand rabbits varying in age from one month to three years, were incubated with 0.1 ng of vitamin B12[57Co] (specific activity: 180 μCi/μg; Lot 247; Radiochemical Centre, Amersham, England) at 37°C for 30 minutes. Starch gel electrophoresis and autoradiography were performed as described by Geldermann (1970) and Daiger et al. (1975b), respectively. Electrofocusing (pH range 3.5–9.5) was conducted in the 2117 Multiphor apparatus (LKB, Bromma, Sweden) according to the manufacturer's instructions. The resulting pH gradient was measured with a surface pH electrode (Ingold, Zürich,