University of California, Davis (UCD) line 200 chickens develop an inherited connective tissue disease which includes fibrosis, vascular occlusion, and lymphocytic infiltration of skin, comb, and viscera. To further identify the nature of these features, tissue from both affected and control chickens, aged 7 days to 12 months, was serially examined using hematoxylin-eosin and Masson's trichrome stain. Mononuclear cell accumulations in skin were simultaneously characterized using mouse anti-chicken T and B cell specific monoclonal antibodies in a solid-phase immunoperoxidase assay; the same cells were also examined by direct immunofluorescence with fluoresceinated anti-chicken IgM and IgG, and stained for esterase and acid phosphatase. At 6 weeks of age, the majority of line 200 chicks manifested dermal fibrosis, cellular infiltrates, and vessel anomalies. In contrast, visceral involvement did not reach maximum incidence until 5 months of age with involvement of small intestine (60%), lungs (60%), and kidneys (65%), and not until 1 year for esophagus (64%), heart (30%), and testes (66%). Moreover, and of particular interest, was the threefold increment of full-thickness skin biopsies in line 200 birds. The cellular infiltrates in skin, found in 100% of affected line 200 chickens, were positive for B1, a mature B cell marker found on peripheral blood and bursal B cells; the majority of these same cells were found to bear surface IgM but not IgG. These data suggest that this syndrome may result from an alteration in collagen metabolism associated with a mature subpopulation of B lymphocytes.