Umbilical cord blood has been used to effect hematological reconstitution and there are sufficient stem cells available in the cord blood obtainable from a single placenta to reconstitute an adult patient. Umbilical cord blood might therefore, have widespread potential use in the field of bone marrow transplantation. We compared alloractivity of paired samples of adult and cord peripheral blood mononuclear cells, by measuring frequencies of both alloraeactive T helper cells and cytotoxic T cell precursors (CTLp), using limiting dilution analysis. In addition we compared the phenotype of adult and neonatal PBMC, using monoclonal antibody staining. Cord PBMC in general showed higher frequencies of alloreactive Th than adult PBMC, with statistically significant differences in 6 out of 10 experiments. There was no statistically significant difference between adult and cord CTLp frequencies. Adult and cord PBMC surface phenotype was similar, except that cord blood contained fewer lymphocyte function-associated-3 (LFA-3) positive (memory) cells.