SummaryThe frequencies of three co‐dominant allelesTfA,TfBandTfccontrolling the serum transferrin locus in native and exotic breeds of pig in the United Kingdom are shown. Matings between different transferrin genotypes and segregation of the transferrin alleles in piglets from 131 litters are also recorded.In eleven litters from matings within a closely related group of animals heterozygous for theTfcallele, no offspring of theTfc/Tfcgenotype were born. Matings between heterozygous related boars believed to be carrying a lethal factor linked to theTflocus and heterozygous unrelated sows of the Saddleback breed, resulted in offspring of theTfc/Tfcgenotype being born.An excess of heterozygote genotypes was found in all litters from matings between animals heterozygous for theTfcallele. In all these litters the average litter size was 0.5 piglets less per litter than the average for all other litters studied. For the eleven litters from matings involving closely related animals heterozygous for theTfcallele, the average litter size was 1.4 piglets less per litter than the average for all other litters.The possibility of an early lethal factor being linked to the Tf locus in this family of pigs is discusse