The monoclonal antibody reactivity of cord blood mononuclear cells to six monoclonal reagents, OKT3, OKT4, OKT6, OKT8, OKT10, and OKT11, was evaluated using an indirect immunofluorescence technique. An immature population of cells was found; 24% of T cells in cord blood expressed OKT6 and 58% expressed OKT10. A previously undescribed phenotype, OKT3+/OKT6+, was discovered using double-labeling studies. In addition, the OKT4+:OKT8+ ratio in cord blood was 1:2, compared with 1:3 in the normal adult controls tested in this study. This is explained by the presence of a population of OKT4+/OKT8+ cells circulating in the neonate. Double-labeling studies of the cord blood samples demonstrated that all of the OKT3+ cells were simultaneously OKT11+ and that 17% of the cells were OKT11+ only. This is in accordance with the findings in normal adult peripheral T cells. Therefore, circulating neonatal T cells express an immature phenotype more characteristic of thymocytes and an altered OKT4+:OKT8+ ratio as compared with adult controls. We also describe a new phenotype OKT3+/OKT6+ present on the cord mononuclear cells.