Mixtures of dipalmitoylphosphatidylcholine with purple membrane lipids, red membrane lipids or total lipids of H. halobium were studied with differential scanning calorimetry. A comparison of red and purple membrane lipids reveals no difference in their phase behavior, indicating that lipid phase behavior plays no role in the in vivo separation of red and purple membranes. The effects of variation of the salt content of the suspending solution were also examined. Studies of the melting behavior of these mixtures as H. halobium lipid content is varied suggest that the gel to liquid-crystal transition does not occur in the lipids of H. halobium.