Recent advances in knowledge of blood groups' polymorphism provide aid to paternity diagnosis. When no exclusion is detected in any of the 20 or more systems available to the laboratory, validation of paternity is recommended. Two indices may be calculated: the proportion of males excluded by the phenotypes of the mother and child, and the probability of paternity of the alleged father. This latter index is based on bayesian process. The different formulations proposed by several authors are compared. The fact that a single exclusion in 20 or more systems needs to be denied or confirmed by indices of paternity is emphasized.