The accumulation of the homozygous plasminogen (PLG) variant A3 in 4 siblings of a family led to the detection of 5 cases of apparent inverse homozygosity of PLG phenotypes which seemed to exclude paternity. Determination of 22 blood group markers and HLA typing, but under exclusion of PLG phenotypes, confirmed paternity in all cases (biostatistical probability of paternity greater than 99.9985%). Comparing the results of 'Western blots' with functional-caseinolytic phenotyping, the existence of inactive plasmin, as described earlier, could be excluded. Besides inverse homozygosity the assumption of a silent allele was confirmed by reduction of PLG antigenic levels and functional activities to approximately 50% of normal range. The PLG phenotype A in 1 individual with anamnestic thrombosis, reduced values of PLG antigen, and reduced functional activity, although in accordance with Mendelian inheritance, was also considered as indicative for PLG hemizygosity.