A study was made of the Mendelian segregation of genes controlling the F-V bloodgroup system and the Tf serum protein system in four cattle breeds. From the parentage verification cases in which both parents and their offspring had been blood typed, cases were selected in which one or both of the parents were heterozygous for one or both systems. In the F-V system a deviation was found from the expected ratios resulting in an increase of the frequency of the gene for the blood-group F in the offspring, in three of the four breeds studied. The explanation that this is caused by a mother-fetus incompatibility was not confirmed by the data obtained on the fourth breed. In the Tf-system, an excess of heterozygous offspring of type AD from AD-type dams and a shortage of homozygous offspring from those dams, compared with the expected numbers, was observed. No explanation for this phenomenon is given, since this material does not permit a detailed study of possible causes. There was no indication of a mother-fetus incompatibility with respect to the Tf types in this material.