Microgel systems (also called microspheres) have a very wide range of possible in-dustrial and biomedical applications. For example, microgels have been studied as delivery vehicle for drug delivers (1), physical-separators (2) or catalysers (3). In all these applications, a particular attention is paid to the affinity of a molecule of inter-est with the microgel. In the present work we present a microgel system showing no specific electrostatic or hydrophobe affinity. We studied the diffusion of various tracers such as proteins (alpha-lactalbumin, beta-lactoglobulin and bovine serum albumin) and dextrans (4, 10, 20, 40, 70 kD) using flu- orescence recovery after photo bleaching (FRAP). Microgels were characterized by trans-mission electron microscopy (TEM) of high-pressure frozen samples. In dense casein solu-tions (> 350-400 g/L) TEM shows that casein forms a continuum medium. FRAP binding analyses (4) demonstrated that there is no significant binding (“purely diffusion" dominant-regime) between the tracer and the gel. More over, tracer diffusion may be readily described using diffusion models of non-sticky particles in an entanglement polymer melt (5, 6). How- ever, in semi-diluted microgel concentrations, FRAP binding analyses indicate a significant affinity between the tracer and the microgel. More importantly, the larger the tracer, the greater is its affinity to the microgel. The cell model of Jonsson (7), which derives a parti-tion coefficient from the effective diffusion of particles in a colloid dispersion, validates the results. Taking into account exclude volumes models (8, 9) we have explained the ability of caseins microgels to trap different tracer thanks to its structure. These results may be im- portant for microgels systems designed to operate in contact to a wide diversity of particles in complex environments (ex. blood plasma).