To determine the phylogenetic relationships among hominoids and the dates of their divergence, the complete nucleotide sequences of the constant region of the immunoglobulin epsilon-chain (C epsilon 1) genes from chimpanzee and orangutan have been determined. These sequences were compared with the human epsilon-chain constant-region sequence. A molecular clock (silent molecular clock), measured by the degree of sequence divergence at the synonymous (silent) positions of protein-encoding regions, was introduced for the present study. From the comparison of nucleotide sequences of alpha1-antitrypsin and beta- and delta-globin genes between humans and Old World monkeys, the silent molecular clock was calibrated: the mean evolutionary rate of silent substitution was determined to be 1.56 X 10(-9) substitutions per site per year. Using the silent molecular clock, the mean divergence dates of chimpanzee and orangutan from the human lineage were estimated as 6.4 +/- 2.6 million years and 17.3 +/- 4.5 million years, respectively. It was also shown that the evolutionary rate of primate genes is considerably slower than those of other mammalian genes.