Methods are reported for the quantitative measurement of pinocytosis in human monocytes isolated from peripheral blood. The cells, in adherent culture in plastic wells, were exposed for periods of up to 48 h to culture medium containing 125I-labelled polyvinylpyrrolidone (50 micrograms/ml) and the pinocytosis enhancer suramin (500 micrograms/ml). Uptake of radiolabel was linear with time and was inhibited by colchicine (100 micrograms/ml), results that are consistent with uptake of radiolabelled substrate by pinocytosis but not with superficial adsorption of radiolabel. Similar results were obtained using a 125I-labelled vinylamine-vinyl-pyrrolidone copolymer as radiolabelled substrate. The rates of pinocytotic uptake of 125I-labelled polyvinylpyrrolidone (in the presence of suramin) and of 125I-labelled copolymer were measured at various stages of in vitro monocyte-to-macrophage maturation. In contrast to an earlier report, we found no consistent differences in pinocytotic activity between cells at different stages of differentiation.