The proportion of exclusion for a given mother-child pair is the proportion of males excluded from the paternity of this child of a known mother and may be calculated given both the child's and mother's phenotypes and the population gene frequencies. Its expected value in the population is equal to the probability of exclusion, which expresses a laboratory's capability to exclude from paternity nonbiological fathers.In a sample of 171 families examined for 20 genetic systems at the National Blood Group Reference Laboratory, 25 exclusions of putative fathers were detected. The ranking by efficiency of the systems used in these exclusions fits the "expectation of their efficiency," and the average proportion of males excluded by the child's and mother's phenotypes is not different from the expected proportion. Additionally, the repetition of exclusions in an incompatible putative father-mother-child trio is not dependent on the overall proportion of males excluded by the mother and the child, but rather on some high values of the proportion of excluded men in some specific systems.Here, formulas and some factors modifying these parameters as well as a more efficient sequence of examinations to exclude paternity than has previously been used are given. Using this sequence, laboratories which carry out several analyses per day can work by levels of five examinations at a time, done in a particular order, to obtain a rather rapid exclusion of certain families.