Three serum transferrin phenotypes of Praomys (Mastomys) natalensis, and African rodent having a high incidence of many varieties of spontaneous neoplasms, were detected by electrophoresis and autoradiography. On electrophoresis, transferrin of Trf-K was found to be a protein that migrates slowly toward the cathode, whereas that of Trf-M migrates rapidly. Trf-KM is a heterozygous phenotype of the above two. Progeny testing demonstrated that these three serum transferrin phenotypes in Mastomys are probably controlled by two codominant allelic genes, Trfk and Trfm, at a single autosomal locus. The genotypes of these phenotypes may be Trfk/Trfk, Trfm/Trfm, and Trfk/Trfm.