Plasma samples from 129 dogs have been typed using an immunofixation technique to define the polymorphism of canine C4. The mode of inheritance was inferred by study of four large pedigrees. In contrast to the situation in man, canine C4 may be encoded by a single locus. At least five different co‐dominant structural alleles have been detected. A deficiency allele has not been found as yet.Because there are associations between C4 allotypes and spontaneous autoimmune disease in man, the well defined polymorphism of canine C4 may provide an excellent model for understanding of the pathogenetic mechanisms of autoimmune disease.