Human synovial tissue cells in monolayer can be shown to take up and digest a soluble protein, horseradish peroxidase (HRP). Uptake of HRP was linear with increasing concentrations of substrate and cell protein and with time for up to 4 h. Low temperature (4 degrees C), and sodium fluoride, an inhibitor of glycolysis were the most effective metabolic inhibitors of endocytosis. In addition, colchicine, an inhibitor of microtubule assembly, and yeast mannan, an inhibitor of mannose-specific receptors, reduced HRP uptake. Synovial cells from patients with rheumatoid arthritis (RSC) demonstrated a statistically significantly higher rate of endocytosis (247 +/- 107 ng HRP/100 micrograms cell protein per 2 h.) than cells from control, nonrheumatoid patients (NSC) (100 +/- 80 ng HRP/100 micrograms cell protein per 2 h). Thus, it is possible to discriminate RSC from NSC by their quantitatively different rates of endocytosis. Digestion of HRP by synovial cells is statistically significant by 6 h after uptake. A faster initial rate of digestion was seen in RSC. Over the first 6--8 h of incubation 42% of the endocytosed HRP was still cell-associated in RSC and 67% remained in NSC cultures. However, by 24 h 20--30% of endocytosed HRP was found in both types of cultures. These results indicate that endocytosed molecules may accumulate more rapidly in RSC and persist within their lysosomes for a longer time than in NSC. The quantitative determination of enhanced endocytosis by RSC compared with NSC suggests that this increased activity may have a role in the pathological function of synovial tissue in rheumatoid arthritis.