Fibroproliferation was measured as the uptake of [3H]thymidine into fibroblasts. Human fibroblasts were incubated with 200 μl monocyte-conditioned medium, the 0.22 μm filtrate from cultured monocytes, in Dulbecco's modified Eagle medium supplemented with controlled process serum replacement 2, a fetal calf serum substitute with low mitogenic activity. Increasing the numbers of fibroblasts resulted in a parallel increase in thymidine uptake to a maximal level. Fibroblasts (2 × 103) were plated into microwell plates and incubated with monocyte-conditioned medium for 72 hr. At 16 hr before harvest, 1 μCi [3H]thymidine was added. Cells were harvested with phosphate-buffered saline and washed, and the filters were counted. Fibroblasts incubated with Dulbecco's modified Eagle medium and controlled process serum replacement 2 showed minimal thymidine uptake. Fibroblasts incubated with Dulbecco's modified Eagle medium plus monocyte-conditioned medium from monocytes stimulated with 10 μg/ml lipopolysaccharides showed a sixfold increase in thymidine uptake over fibroblasts in Dulbecco's modified Eagle medium and controlled process serum replacement 2 alone. Fibroblasts incubated with Dulbecco's modified Eagle medium plus monocyte-conditioned medium from monocytes of patients with liver disease (n = 20) showed a 10-fold elevation in thymidine uptake compared with Dulbecco's modified Eagle medium and controlled process serum replacement 2. Results indicated that preincubation of monocyte-conditioned medium with either anti-interleukin-1β (12.5 halfmaximal units, 4° C, 16 hr) or catalase (1,870 IU, 25° C, 1 hr) did not alter the fibroproliferative activity of the monocyte-conditioned medium, suggesting that neither interleukin-1β nor activated oxygen intermediates were involved in fibroproliferation. Fibroblasts were incubated with platelet-derived growth factor in increasing concentrations to produce a dose-response relationship. Platelet-derived growth factor was found to significantly enhance fibroproliferation. The addition of antibody to platelet-derived growth factor reduced the fibroproliferation activity of plateletderived growth factor. Samples of monocyteconditioned medium were then preincubated with anti—platelet-derived growth factor to determine whether platelet-derived growth factor in the monocyte-conditioned medium was mediating fibroproliferation. Anti—platelet-derived growth factor reduced the fibroproliferative activity of the monocyteconditioned medium, suggesting that platelet-derived growth factor probably plays a role in the fibroproliferation observed with monocyte-conditioned medium obtained from patients with liver disease. (HEPATOLOGY 1992;15:191–197).