The capacity of human cord blood (CB) lymphocytes to form T-colonies was studied with a double layer technique. The mean number of colonies in CB was 91 +/- 70.5 SD (X 10(5) cells), significantly lower than in adult blood, mean 182 +/- 58.0 (X 10(5). In 28 of the 50 CB samples tested the colony numbers were below the normal range for adult lymphocytes. There was no direct correlation between number of colonies and percentage of E-rosette-forming cells in CB. Some CB samples with a high proportion of E-rosettes formed few T-colonies, suggesting that not all E-rosette positive cells are capable of producing T-colonies. On the other hand, some CB samples with a low proportion of E-rosettes formed normal numbers of T-colonies. Purification of two populations of T-cell enriched and T-cell depleted lymphocytes confined the T-colony growth in CB, as in adult blood, to the former fraction, excluding that T-colonies could be obtained from E-rosette negative lymphocytes. This indicates that, from birth onwards, T-lymphocyte colonies originate from E-rosette positive cells. Whether the low growth observed in CB results from lack of maturation of T-lymphocytes or from the presence of specific subsets of T-lymphocytes is not clear at the present time.