We used the human/sheep competitive engraft model to investigate the in vivo engraftment potential of human CD34-, Lin-cells. In three separate studies, transplantation of CD34+, Lin-and CD34-, Lin-cells from the same normal human donors into preimmune fetal sheep resulted in long-term engraftment and multilineage hematopoietic cell/progenitor expression. Human cell/progenitor activities of the CD34+ group were CD3, 0.52%; CD13, 0.36%; CD45, 1.02%; glyA, 1.78%; HLA-DR, 0.99%; colony-forming units (CFU)-Mix, 7.1%; and granulocyte-macrophage CFU (CFU-GM), 13.7%; those of the CD34-group were CD3, 0.24%; CD45, 2.49%; glyA, 0.75%; HLA-DR, 0.3%; CFU-Mix, 7.1%; and CFU-GM, 4.3%. Of special interest was the detection of highly significant numbers (1 x 10 [9] cells) of human CD34+ cells in animals transplanted with CD34-cells. Secondary transplantation and limiting dilution studies confirmed the presence of cells with long-term engraftment potential in CD34-populations. The results presented here demonstrate that the CD34-fraction of normal human bone marrow contains cells capable of engraftment and differentiation into CD34+ progenitors and multiple lymphohematopoietic lineages in primary and secondary hosts.