The lateral diffusion of the excimer-forming probe pyrene decanoic acid has been determined in erythrocyte membranes and in vesicles of the lipid extracts. The random walk of the probe molecules is characterized by their jump frequency, nu j, within the lipid matrix. At T = 35 degrees C a value of nu i = 1.6 . 10(8) s-1 is found in erythrocyte membranes. A somewhat slower mobility is determined in vesicles prepared from lipid extracts of the erythrocyte membrane. Depending on structure and change of the lipids we obtain jump frequencies between 0.8 . 10(8) s-1 and 1.5 . 10(8) s-1 at T - 35 degrees C. The results are compared with jump frequencies yielded in model membranes. The mobility of molecules perpendicular to the membrane surface (transversal diffusion) is investigated. Erythrocyte ghosts doped with pyrene phosphatidylcholine were mixed with undoped ghosts in order to study the exchange kinetics of the probe molecule. A fast transfer between the outer layers of the ghost cells tau 1/2 = 1.6 min at T = 37 degrees C) is found. The exchange process between the inner and the outer layer of one erythrocyte ghost (flip-flop process) following this fast transfer occurs with a half-life time value of t 1/2 = 100 min at T = 37 C. The application of excimer-forming probes presumes a fluid state of the membrane. Therefore we investigated the phase transition behaviour using the excimer technique. Beside a thermotropic phase transition at T = 38 degrees C and T = 33 degrees C we observed an additional fluidity change at T = 38 degrees C in erythrocyte ghost. This transition is attached to a separation of the boundary lipid layer from the intrinsic proteins. No lipid phase transition is observed in liposomes from isolated extracts of the erythrocyte membrane with our methods.