A convenient method was devised to chromatograph simultaneously numerous samples of reassociated DNA under thermoregulated conditions. Small amounts of hydroxyapatite (0.2-ml bed) were used and the effluent volumes for separating single-stranded DNA from DNA duplexes were 1.5 ml each. The method was comparable in reliability and reproducibility to other well-established techniques, but it combined improvements in ease of operation, rapidity and simplicity, which are desirable in large taxonomic studies [of bacteria].