The mammary gland is of cutaneous origin with stromal leucocytic infiltrates and it excretes leucocytes in the milk. Milk T lymphocytes are highly enriched by sedimenting milk somatic cells in a 43 per cent preformed Percoll gradient. Thirty to 54 per cent of leucocytes, harvested from the upper Percoll band, belong to CD2+ CD3+ T lymphocytes These milk T lymphocytes are mature CD4+, or CD8+ single positive T cells, expressing CD44, CD26, and B1 integrin, but not Lam-1. Milk T lymphocyte subsets may play an important role in regional immunosurveillance of the mammary epithelia and stroma. Selective mechanisms may account for the trafficking of this activated, memory T lymphocyte subset in the gland.