Abstract The interactions of benzo(a)pyrene (B(a)P) with the cell surface membrane were studied by measuring B(a)P uptake into intact mammalian cells and by determining B(a)P fluorescence in the presence of isolated cell surface membranes. It was found that 0.19 μg B(a)P were taken up by 10 6 Chinese hamster ovary (CHO) cells after 30 min exposure to a solution containing 0.59 μg/ml. Culture conditions were found to markedly alter B(a)P uptake. Low cell culture densities resulted in a fourfold increase in rate of B(a)P uptake per cell relative to confluent monolayer cultures. The uptake rate of B(a)P was reduced in the presence of bovine serum (BS) and, under some conditions, perylene. This information should be considered in the design of experiments on the biological effects of B(a)P. Another aspect of B(a)P membrane interaction was that the binding of B(a)P to cell surface membranes could be measured by fluorescence. The additional B(a)P fluorescence, found in the presence of cell surface membranes, was sufficiently large that the methods of data treatment used in the study of fluorescent probe-membrane interactions could be applied to get quantitative information on B(a)P-membrane interactions. It was found that 0.6 · 10 −8 moles B(a)P were bound per mg membrane protein and that the apparent statistical dissociation constant for the complex was 3.8 · 10 −7 M. The data suggest that the mechanism of uptake of B(a)P is probably passive diffusion.