Fourier transform infrared spectroscopy is applied to the study of intact and deproteinated plasma membranes of Acholeplasma laidlawii, enriched biosynthetically with perdeuteriopalmitoyl chains. The temperature-dependent behavior is monitored via the CD stretching modes and compared with that observed in the model membrane 1,Zdiperdeuteriopalmitoyl-sn-glycero-3-phosphocholine. The broad ramplike transition observed in the natural membranes is shown to consist of two overlapping stages. In the lower temperature range the principal change is a reduction in the rigidity of the lipid matrix. Subsequently, over a 5 ºC range, centered at the growth temperature, a large change in the gauche/trans conformer ratio of the acyl chains occurs, similar to that generally observed in model systems. This is followed by an abrupt cessation of the phase transition. The effects of membrane protein on the phase transition are shown to be relatively minor. Firstly, they produce a decrease in the rate of acyl chain motion at a given temperature, resulting in a reduction in the width of the transition. In addition, the presence of protein increases the population of gauche conformers of the fatty acyl chains in the liquid-crystalline phase of the membrane lipids.