We have compared the number of copies of sequences complementary to a cloned Xenopus histone H4 coding sequence in the genomes of Xenopus, Triturus and Ambystoma, amphibian species with widely different C-values (3, 23 and 38pg DNA/haploid genome respectively). Quantitative autoradiography indicates that H4 sequence constitute a greater proportion of the genome the larger that genome is. Measurement of the absolute copy-number by reassociation kinetic analysis indicated 47 +/- 10, 636 +/- 21 2685 +/- 349 copies per haploid genome each in Xenopus, Triturus and Ambystoma respectively. Whilst this confirms a trend of increasing copy-number with increasing C-value, the two are not directly proportional and some other factors must contribute to determining the number of copies of these genes.