University of California, Davis (UCD) line 200 chickens spontaneously develop a progressive fibrotic syndrome with features similar to those observed in human autoimmune connective tissue diseases, including fibrosis, vascular occlusion, and lymphocytic infiltration of the comb, skin, digits, and viscera. Beginning at 2 weeks post hatch, line 200 chickens develop intense lymphocytic infiltration of the comb and dorsal neck skin. To further characterize the nature of these cellular infiltrates, weekly serial skin biopsy specimens from line 200 and control birds were examined using hematoxylin and eosin staining and indirect immunofluorescence with a library of mouse anti-chicken monoclonal antibodies specific for lymphocyte markers. In situ staining performed on serial skin sections revealed the presence of large groups of T cells beginning at 2 weeks of age. Further characterization of these infiltrates demonstrated the presence of both T helper and T cytotoxic/suppressor cells with a mean +/- SD T4:T8 ratio of 1.44 +/- 0.29 by 4 weeks of age. As the lesions progressed, the infiltrates also contained distinct groups of B cells as characterized by MUI 36. In addition, the lesions were strongly positive for B-L (Ia) antigen, which was noted on B cells, monocytes/macrophages, activated T cells, and fibroblasts. The skin sections were negative for 2 different macrophage monoclonal antibodies at all time-points. Upon extraction from affected skin, 42.0 +/- 13.06% (mean +/- SD) of these cells were positive for B-L, 35.10 +/- 6.51% were T cells, and 31.25 +/- 3.14% were recognized by MUI 36. Although positive staining for IgG was not found in these extracted cells, 7% of the isolated cells were positive for surface IgM.