Comparative Determinant Analysis of 69 primate plasma proteins reveals 321 antigenic determinants for phylogenetic inference. These determinants, which are discrete characters with innate phylogenetic polarity, suggest paraphyletic cladogenesis of strepsirhine prosimians and of New World monkeys, and firmly establish the chimpanzee as man's closest relative. Divergence dates of primate clades are estimated by the molecular clock approach.