In previous autoradiographic studies on the sites of catabolism of rat serum albumin (RSA) in the rat, fibroblasts in skin and muscle were shown to accumulate degradation product from RSA labeled with the residualizing label dilactitol-'251tyramine ('251-DLT) (Strobel et al., 1986 J. Biol. Chem., 261:7989-7994). Residualizing labels remain at the cellular site of degradation of the carrier protein because of their size, hydrophilicity, and resistance to lysosomal hydrolases. This study was designed to evaluate whether fibroblasts might retain labeled degradation products more efficiently than other cell types. The uptake of '"I-DLT-RSA and release of its degradation products and of a second non-biodegradable probe, fluorescein isothiocyanate (FITC)-dextran, were studied in fibroblasts, endothelial cells, and macrophages, all cell types previously implicated in the catabolism of albumin in vivo. The rates of uptake of labeled protein and dextran were comparable in all cell types and consistent with fluid phase endocytosis. The rate of release of both intact protein (30-35% of total radioactivity released) and radioactively labeled degradation products followed similar kinetics and had half-lives ranging from 26 to 37 hr. The rate of release of FITC-dextran was slower than that of radioactivity, with a half-lifeof42-125 hr. Thus, although there weredifferences between the rates of release of the fluorescent and radioactive materials in vitro, there were no significant differences in the disposition of protein-derived catabolites among these three cell types.