Erythrocytes of Salmo irideus trout were separated in the range from 45 to 65% Percoll, yielding three well-separated different fractions. Steady-state fluorescence of probes embedded in erythrocyte membranes and/or in liposomes from extracted lipids was used to characterize their physicochemical properties. Furthermore, the fluorescence decay of 1,6-diphenyl-1,3,5-hexatriene (DPH), embedded in the same liposomes, was measured by a frequency decay fluorometer. DPH decay was analyzed on the assumption of continuous distribution of lifetimes, for evaluating modifications of membrane microheterogeneity. Significant differences were observed in the parameters measured for the three erythrocyte fractions, possibly connected with the specific lipid composition of the samples.