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The lateral diffusion of a fluorescent phospholipid probe in oriented multibilayers of dimyristoylphosphatidylcholine has been measured by observing the redistribution of fluorescence after photobleaching of the membranes in a periodic pattern of parallel stripes. The diffusion constant D of the fluorescent lipid was found to vary between 1.5 X 10(-11) cm2/sec at 9.6 degrees and 2.0 X 10(-10) at 22.5 degrees in the monoclinic phase. Preliminary studies of dipalmitoylphosphatidylcholine liposomes in the Lbeta' and Pbeta' phases yielded diffusion constants of the order of 10(-11) cm2/sec. These data are relevant to earlier discussions of the rate of complement activation by hapten-sensitized liposomal membranes [Brûlet, P. and McConnell, H. M. (1976) Proc. Natl. Acad. Sci. USA 73,2977--2981; Parce, J. W., Henry, N. and McConnell, H.M. (1978) Proc. Natl. Acad. Sci. USA 75, 1515--1518)]. We have also used this …