The generation of human T-lymphocyte colonies from different lymphocyte subpopulations in the presence of PHA alone, or PHA plus media conditioned by PHA-stimulated lymphocytes (PHA-LCM) has been investigated. The separation technique consisted of phagocytic cell depletion by carbonyl iron treatment and fractionation of non-phagocytic cells (NP cells) into B cells and T + null cells by affinity chromatography on an anti-F(ab')2 column. The T cells were separated from the null cells by E-rosette sedimentation. Under these conditions, we showed that: no T-lymphocyte colonies were obtained from the null-cell subset in the presence of PHA of PHA + PHA-LCM; T-lymphocyte-colony formation potential was retained in the T-cell subset. Some variability was observed in the production of T colonies using peripheral blood lymphocytes (PBL) from different donors. Low producers and high producers of T-lymphocyte colonies were encountered. The low production of T-lymphocyte colonies observed in some donors was due to a suppressive effect mediated by the phagocytic cells, probably monocytes. The anti-F(ab')2 immunoadsorbent retained a cell population necessary for T-lymphocyte colony growth.