Collagen-producing, marrow-derived adherent cells (MDAC) provide a microenvironment which supports haemopoietic stem cell (HSC) proliferation in vitro. To investigate the relationship between collagen-producing cells in intact marrow and in MDAC cultures, quantitative studies of collagen synthesis were performed. Collagen synthesis by fresh murine bone marrow cells was 0.22% of total protein synthesis and collagen types I and III were synthesized in a ratio of approximately 5:1. During MDAC cultivation, the collagen synthesis fraction increased to 2.3%. The relative amounts of type I and type III remained more or less constant. The MDAC monolayers showed characteristic pleomorphism, and were shown to support HSC proliferation in continuous bone marrow culture for periods of at least 5 weeks. These results suggest that MDAC culture, under the conditions reported, selects a specific type of bone marrow connective tissue cell, probably a reticulum cell, which synthesizes collagen types I and III in a ratio of approximately 5:1.