The presence of T-lymphocytes in mixed hematopoietic colonies (CFU-MIX) has been reported by some investigators. Though depletion before culturing was performed, residual T cells might be responsible for the observed phenomenon. Using nondepleted marrow or bone marrow depleted to about 2%, T-lymphocytes could be detected in mixed colonies. However, reduction of the T-lymphocytes to less than 0.7% by using a modified E-rosette technique or a cocktail of anti-T-cell monoclonal antibodies (WT1, WT32, WT82) in the presence of baby rabbit complement, resulted in mixed colonies free of T-lymphocytes. After addition of 1.75% T-lymphocytes to this T-cell-depleted bone marrow, T-lymphocytes could be detected in most mixed colonies, but not after the addition of the same percentage of irradiated T-lymphocytes. The presence of T cells in mixed colonies was determined by an adapted immunofluorescence technique (WT32 plus GAM-FITC). The results indicate that mononuclear cells with T-lymphocyte antigens are not the offspring of mixed hematopoietic colony-forming progenitors, but of a low number of T-lymphocytes contaminating the bone marrow after insufficient T-cell depletion.