Lateral diffusion of bacteriorhodopsin and a lipid analogue has been measured in dimyristoylphosphatidylcholine bilayers as a function of temperature, phospholipid/protein (mol/mol; L/P) ratio, and aqueous phase viscosity. The protein lateral diffusion coefficients measured above the temperature at which the lipid gel-liquid/crystalline phase transition occurs (Tc) are combined with previously determined rotational diffusion coefficients to provide a test of the Saffman-Delbrück equations [Saffman, P. G. & Delbrück, M. (1975) Proc. Natl. Acad. Sci. USA 72, 3111-3113]. Insertion of the diffusion coefficients into these equations enables the protein diameter to be calculated. The value of 4.3 +/- 0.5 nm so obtained is in reasonable agreement with the known structure of bacteriorhodopsin. A 12-fold increase in the viscosity of the aqueous phase reduces protein lateral diffusion coefficients by 50%, which is also consistent with the Saffman-Delbrück equations. Both protein and lipid lateral diffusion coefficients decrease with decreasing L/P ratio above the Tc. It is argued that, at a high L/P ratio, this effect is probably due to changes in membrane viscosity while, at a low L/P ratio, "crowding" effects (steric restrictions) and protein aggregation become important. When comparing diffusion measurements made in different systems, it is important to take the effect of the L/P ratio into account. When this is done, other published measurements of freely diffusing membrane proteins are in good agreement with the present results and the predictions of the Saffman-Delbrück equations. Below the Tc, the presence of protein enhances diffusion rates. The overall effect is to smooth out the large change in diffusion coefficient that occurs at the Tc.