A new technique for the culture of T-lymphocytic colonies is reported. The method may be regarded as a human lymphocyte precursor cell assay, as is the myeloid colony culture for granulocyte-macrophage progenitors. The colonies arise under the simultaneous stimulation of phytohemagglutin and a leukocyte feeder. A linear relationship is found between colony numbers and cell numbers plated. The colonies represent aggregates of lymphoblast-like cells, the majority of which are capable of E-rosette formation, are responsive in mixed lymphocyte cultures, and do not exhibit surface immunoglobulins. Their density distribution profile is very similar to that of myeloid colony-forming cells. The finding that most of these colony-forming cells are recovered in the so-called lymphocyte-free stem cell fraction following density fractionation suggests that they originate from a lymphocytic precursor.