Primary cell cultures from the livers of mice infected with Schistosoma mansoni were prepared and cells with the appearance of fibroblasts by light microscopy were isolated. Collagen synthesis was estimated by measuring incorporation of 14C-proline into collagenase-sensitive proteins for both culture media and cell layers. Coculture of splenic T cells from infected mice with these hepatic fibroblasts caused greater selective and specific reduction in collagen production than did coculture using spleen cells from normal mice. There was a parallel inhibition in collagen within the cell layer which indicates that the marked decrease in collagen production was due to inhibition of synthesis and not related to changes in solubility or secretion. Primary culture of mouse skin fibroblasts showed similar responses to coculture but an established fibroblast line, 3T3, was unresponsive. Inflammatory cells appear to influence hepatic fibroblasts isolated under our experimental condition in several ways, such as opposite effects on collagen synthesis and cell proliferation.