the critical electrolyte concentrations (cec) of sperm chromatin from animal species known or suspected to contain histone h1 variants were compared by examining the affinity of their dna-protein complexes for toluidine blue in the presence of mg2+. bullfrog, sea urchin, bee and bumblebee spermatozoa were studied. the cec for rana catesbeiana and two sea urchin species were similar to that of histone h5-containing chromatin from chicken erythrocytes, thus confirming the biochemical and structural similarities of these dna-protein complexes. the cec for bees and the bumblebee, bombus atratus, showed no particular phylogenetic relationship. we concluded that the cec of histone h1-containing sperm cell chromatin is a useful indicator of variability in dna-protein complexes but is of little phylogenetic value.