We investigate the lateral diffusion of Human Serum Albumin-Fluorescein Isothiocyanate (HSA) adsorbed "on" or embedded "in" poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH) multilayers. Special attention is brought to the evolution of the diffusion coefficient with the surface HSA concentration. We find that while on PSS terminating films the diffusion coefficient of adsorbed HSA is independent of the protein surface concentration in the explored range. it decreases strongly with the surface concentration when HSA is adsorbed on PAH ending films. On both films, the mobile fraction of adsorbed protein molecules decreases when the surface concentration increases. At low surface coverage, up to 90% of the adsorbed protein molecules are mobile both on PSS and PAH terminating films. The decrease of the mobile fraction with the HSA surface concentration is more pronounced on PSS than on PAH reaching, respectively, 50% and 70% of mobile HSA molecules at high surface coverage. This behavior is typical for protein aggregation. Infrared spectroscopy in the ATR mode confirms the presence of protein interactions but also rules out that this constitutes the unique reason for the evolution of the mobile fraction with the surface coverage. We also find that the diffusion coefficient, at small surface concentration. is more than I order of magnitude smaller on PSS than on PAH ending films, the diffusion coefficients being, respectively, equal to 6.2 x 10(-11) cm(2)/s and 2 x 10(-9) cm(2)/s. A tentative model based on the wrapping of HSA molecules by PAH chains and bridging between the chains by both polyelectrolytes is proposed to explain the observed features. Finally, we also determine the diffusion coefficient of HSA embedded in PSS/PAH multilayers. We find that the diffusion coefficient of HSA embedded in -PAH-HSA-PAH-type multilayers is close to that determined when HSA is adsorbed on PAH terminating films. When HSA is embedded in -PSS-HSA-PSS-type films the diffusion coefficient is independent of the HSA surface concentration and is surprisingly slightly larger than when HSA is adsorbed on PSS terminating films, 1.1 x 10(-10) cm(2)/s compared to 6.2 x 10(-11) cm(2)/s.