1. 1. The influence of the fatty acid composition of membrane lipids on the quantitative distribution of membrane protein of Mycoplasma laidlawii has been investigated. 2. 2. Cells were grown in media supplemented with either a saturated fatty acid (palmitic acid or stearic acid) or an unsaturated fatty acid (palmitoleic acid or oleic acid). Lipids isolated from membranes of these cells were markedly enriched in the supplemented fatty acid. 3. 3. The proteins of the different types of membranes were analyzed by sodium dodecyl sulfate-acrylamide-gel electrophoresis of membranes from cells grown in the presence of radioactively labelled amino acids. In general, protein composition and relative abundance were unaffected by fatty acid variation. One high molecular weight protein fraction was present in higher concentration in membranes rich in saturated fatty acids than in those rich in saturated fatty acids. 4. 4. The membranes rich in saturated fatty acids were also found to have a slightly higher density as determined by isopycnic centrifugation. © 1972.