We have studied peripheral blood stem cells (PBSC) collected by cytapheresis following intensive chemotherapy, from 13 patients with acute leukemia, in long term culture (LTC). Peripheral blood was cultured with (n = 10) and without (n = 21) the addition of a preformed, irradiated stromal layer. In this latter LTC our results confirm that peripheral blood is capable of producing CFU-GM and nucleated cells in the absence of the formation of an adherent stromal layer. However, peripheral blood cultured in the presence of an irradiated stromal layer is capable of a significantly higher proliferative response (total production of CFU GM per flask - mean = 57529) than in the absence of an irradiated stromal layer (total production of CFU GM per flask - mean = 26739, p less than 0.03). Our results suggest that PBSC contain a primitive nonplastic adherent cell that requires the presence of a stromal layer for its expression. These findings provide further support for the use of peripheral blood stem cells for autologous transplantation.