The interaction of native and cationized bovine serum albumin-gold complex with confluent monolayers of goat [corrected] brain microvascular endothelial cells was investigated. These cells were used as an in vitro model of the blood-brain barrier. After exposure to both complexes for 5, 15, 30, and 60 min, the endothelial cells were washed with phosphate-buffered saline, fixed, and processed for electron microscopy. The results obtained suggest that (a) contrary to previous observations in vivo, cultured endothelial cells were able to transfer native bovine serum albumin-gold complex particles from apical to basolateral surfaces presumably via a fluid-phase mechanism; (b) the most conspicuous difference in the interaction of both albumin-gold complexes consisted in a significantly greater adsorption of cationized than of native bovine serum albumin-gold complex to the apical surface of the endothelial cells; (c) the further fate of the endocytosed complexes was similar, that is, their major part was internalized into endosomes, multivesicular bodies and lysosomes whereas a smaller fraction was apparently transferred to the basolateral plasma membrane where presumably they were exocytosed into the subendothelial space. These observations provide new ultrastructural evidence that both native and cationized albumin are endocytosed and eventually transported by a non-specific, fluid-phase mechanism, and also support the results of earlier quantitative studies indicating the absence of albumin receptors in brain endothelia.