Binary monolayers of L-alpha-dilauroylphosphatidylcholine (DLPC) and cholesterol were formed at the air/water interface by the successive addition method with chloroform as the spreading solvent at 296 K. Translational diffusion of a probe lipid in the monolayers as a function of cholesterol fraction was examined by the technique of fluorescence recovery after photobleaching in conjunction with epifluorescence microscopy and the Wilhelmy plate method. A transition from biphasic morphology to a visually homogeneous uniphasic monolayer occurred at surface pressures between 13 and 20 mN.m(-1), depending on the cholesterol fraction in the monolayer. The lateral diffusion coefficient was found to decrease when surface pressure and/or cholesterol fraction were/was increased. The diffusion coefficients of the probe lipid in the homogeneous monolayers were analyzed on the basis of the free area model. The observed retardation of the diffusion is ascribed to an increase in the in-plane surface viscosity of monolayers. In the case of biphasic binary monolayers, the retardation with increasing cholesterol fraction is attributed to the obstacle mediated diffusion as in the effective medium analysis as well as to increasing surface viscosity.