SummaryOn the basis of the frequency of blood group alleles in Danish Landrace pigs, the theoretical probability of excluding incorrectly alleged paternity and parentage by means of the various blood group systems are calculated. The probability of excluding incorrect paternity of 1 and 4 pigs, will be 85 and 97%, respectively. In 96% of the cases, the parentage of an accidentally interchanged pig can be excluded.A study of the progeny groups actually excluded during the period 1960‐1968, showed good agreement between the observed and theoretically expected frequencies of exclusions in the various blood group systems, except for the G‐sys