Fibroproliferation was measured as the uptake of [3H]thymidine into fibroblasts. Human fibroblasts were incubated with 200 microliters monocyte-conditioned medium, the 0.22 microns 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 x 10(3] were plated into microwell plates and incubated with monocyte-conditioned medium for 72 hr. At 16 hr before harvest, 1 muCi [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 micrograms/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-1 beta (12.5 half-maximal units, 4 degrees C, 16 hr) or catalase (1,870 IU, 25 degrees C, 1 hr) did not alter the fibroproliferative activity of the monocyte-conditioned medium, suggesting that neither interleukin-1 beta nor activated oxygen intermediates were involved in fibroproliferation.(ABSTRACT TRUNCATED AT 250 WORDS)