Lateral mobilities of lectin receptors and surface immunoglobulins were measured in plasma membranes of hepatocytes prepared by smearing small pieces of rat liver tissue and then using the fluorescence recovery after photobleaching (FRAP) technique. Smears were treated with various doses of fluorescein isothiocyanate (FITC) conjugated concanavalin A (ConA), succinylated ConA (SConA), wheat germ agglutinin (WGA), and soybean agglutinin (SBA), as well as with rabbit anti-rat IgG (RARa/IgG) and goat anti-rat IgM(Fc) (GARa/IgM(Fc] antisera. 10 micrograms/ml ConA and SConA concentrations and a 55 X dilution of the GARa/IgM(Fc) antiserum were found to be suitable for measuring the lateral mobilities dependent on age. Diffusion constant and mobile fractions of receptor complexes were measured in different age groups of female Fisher rats (from 1 to 26 month-old). The FRAP measurements revealed that at least two major receptor sites can be distinguished in cell membranes of compact tissue (similar to the cultured and isolated cells), forming a mobile and an immobile fraction. The mobile fractions of both the lectin receptors and the surface immunoglobulins tended to decrease with age, while the age differences of the diffusion constants were not statistically significant. The observed alterations could be due to the covalent crosslinking of the mobile receptors to immobile patches and/or to the retardation of free diffusion by the cytoskeleton, dependent on age.