In a study of T cell ontogeny using monoclonal antibodies specific for chicken T cell receptors (TcR) and associated cell surface molecules, we found a subset of lymphocytes that express cytoplasmic CD3 epitopes in the absence of cell surface CD3/TcR complexes. Approximately half of these cells, which were present in the spleen, bursa and intestine of young chick embryos, expressed surface CD8. None expressed CD4, TcR 1 (gamma/delta), TcR 2 (alpha/beta) or TcR 3 (a third CD3-associated heterodimer in the chicken). These cytoplasmic CD3+CD8 +/- cells, tentatively named TcR 0 cells to denote their lack of surface TcR, appeared first in the spleen of 8-day embryos, 4 days before TcR expression begins in the thymus, and reached a peak frequency of approximately 10% of the splenic cell in 14-day embryos. The TcR 0 cells were also present in adult birds, where they comprised only about 1% of the CD3+ spleen cells and approximately 40% of the lymphocytes in the intestinal epithelium. We conclude that the avian TcR 0 cells represent a thymus-independent lineage of lymphocytes which, like natural killer cells in mammals, may play an important role in body defense.