Avian T cell development has been traced using monoclonal antibodies specific for chicken T cell surface differentiation antigens. The basic antigen recognition molecules, differentiation pathways, and functional capabilities defined for T cells in mammals are highly conserved in birds. Novel features of T cell development are also revealed. These include the generation of a third T cell sublineage that expresses a different receptor isotype (TCR3) and a novel lineage of cytoplasmic CD3+ lymphoid cells (TCR0 cells). In contrast to the TCR0 cell lineage, the sublines of T cells bearing the three TCR isotypes are all generated exclusively within the thymus. © 1990 by W.B. Saunders Company.