SummaryColonies of human lymphocytes with T cell characteristics will grow in agar from repeated mitotic divisions with phytohaemagglutinin (PHA) stimulation. The colonies comprise spheres of tightly‐packed cells with up to 500–1,000 blast‐like cells in each colony. 65% of cells from pooled colonies bound AET‐treated sheep red cells. 1,100–2,500 colonies/106peripheral blood lymphocytes developed when cell donors were healthy but lower numbers (350–1,000 colonies/106lymphocytes) were detected in blood from cancer patients. Comparison with other non‐specific assays of cell‐mediated immunity showed that, while 66% of cancer patients were anergic (to five recall antigens) and 78% exhibited depressed mitotic activity in standard cultures with low dose PHA, 100% of these patients revealed T cell colony formation below normal. It is suggested that further studies of T lymphocyte colony‐forming cells in healthy people and in a number of disease states may significantly advance our understanding of mechanisms of cell‐mediated immunity.