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Misidentification of sires of cows in paternal half-sib analyses of variance biases the estimate of heritability downward. Four models o£ misidentification yield approximately the same reductions in estimated heritability. The reduced estimates are approximately p2 of the actual heritability where p is the fraction of cows whose sire is correctly identified. The reduction in heritability estimates from daughter-dam regression would be to a fraction, p, of the actual heritability where p is the fraction of dams correctly identified. Empirical paternal sib analyses of New York Dairy Herd Improvement Holstein records suggest that misidentification is more common among grade cows with registered dams and grade cows having grade dams which are not identified than among registered cows with registered dams.