AbstractT‐cell colonies derived from peripheral blood mononuclear cells plated in agar wtih phytohemagglutinin (PHA) contain about 5‐7% E‐, OKT3‐, OKT6‐, TdT‐, SIg‐cells phenotypically similar to bone marrow precursors since they are also RFBl+, OKTIO+, HLA‐DR+, and PNA+. These cells can generate secondary E+, OKT3+T‐cell colonies when replated in the presence of both PHA and a colony‐promoting activity distinct from the T‐cell growth factor, Interleukin 2. These observations suggest that E‐, OKT3‐cells detected in primary PHA‐induced T‐cell colonies belong to a pool of prethymic cells which are able to mature into T‐lymphocytes of T4 and T8 type in the absence