An investigation was carried out on the in vitro effect of a calf thymus extract, thymostimulin, on the distribution of T cell phenotypes as defined by OKT3, OKT4, and OKT8 murine monoclonal antibodies and on E-rosetting cells in human cord blood lymphocytes from healthy newborns. The percentages of E-rosette-forming lymphocytes and OKT3+ total T population were lower in newborns than in adults (E-rosettes: 43.8% +/- 13% vs 57.9% +/- 7.9%, p less than 0.01; OKT3+ cells: 53.3% +/- 15.5% vs 79.9% +/- 4.7%, p less than 0.01), while the OKT4+/OKT8+ (helper/suppressor) cell ratio was normal in both (newborns: 3.40; adults: 2.44-NS). Thymostimulin increased the number of E-rosette-forming cells from 43.8% +/- 13% to 49.9% +/- 12.7% (p less than 0.01), as well as the percentage of phenotypic T lymphocytes. The highest increases were observed in the OKT4+ cells (37.7% +/- 14% to 49.1% +/- 13.8%, p less than 0.001), while smaller changes were observed in the OKT3+ cells (53.3% +/- 15.5% to 58.1% +/- 13.3%, p less than 0.02) and OKT8+ cells (12.8% +/- 6.4% to 16.6% +/- 6.5%, p less than 0.02). The results of the present study suggest that thymostimulin mainly provokes an increase in the helper T cell phenotype in cord blood lymphocytes.