Human cord blood is an attractive alternative to marrow-derived stem cells for transplantation. Experiences with cord blood transplants suggest that graft-versus-host disease (GvHD) may be less readily induced, even in the face of HLA differences. However, this decreased potential for GvHD might also abrogate the graft-versus-leukemia (GvL) effects of the transplant. The GvL potential might be doubly compromised since cord blood NK activity is also decreased. We have compared alloreactivity, NK cell activity and lymphokine-activated killer cell (LAK) activity of cord blood mononuclear cells with adult mononuclear cells. We find a reduced (but not absent) alloproliferative, allostimulatory and allocytotoxic capacity of cord blood mononuclear cells. Phenotyping revealed no significant differences in the proportion of T cells in cord-versus-adult blood, but cord blood T cells were nearly all of the naive CD45RA subset. Expression of LFA-1 alpha and LFA-1 beta was normal on resting cord T cells; however, they expressed significantly less ICAM-1 (CD54) than did adult PBMC. Cord blood B cells and monocytes expressed normal levels of HLA Class II. Although no differences were found in NK cell percentages or subsets in resting cord blood, cord blood NK activity was very low. However, LAK activity was much more readily induced in cord blood as compared to adult PBMC, which could be explained in part by a higher frequency of LAK precursors (LAKp). Cord blood LAK cells were readily able to lyse fresh leukemia targets from patients with ALL, AML, and CML.(ABSTRACT TRUNCATED AT 250 WORDS)