AbstractThe study was undertaken to elucidate some functional characteristics of T cell subsets in human cord blood. A comparison of the cellular interactions involved in thein vitroregulation of pokeweed (PWM)vs. Epstein‐Barr virus (EBV)‐driven B cell differentiation was donein vitroin short‐term cultures of lymphocytes from newborns or adults. T cell subsets were isolated using the monoclonal antibodies OKT4+and OKT8+.OKT4+but not OKT8+T lymphocytes from adults as well as neonates suppressed EBV‐induced immunoglobulin (Ig) secretion of B lymphocytes from adults. This inhibition was mediated through gamma‐type interferon (IFN‐γ). B cells from newborns were not inhibitable by OKT4+lymphocytes as a result of their insensitivity to IFN‐γ.Helper activity for PWM‐induced Ig secretion was exclusively contained within the OKT4+population from adult T cell donors. This function was normally not detectable in any of the neonatal T cell subsets. OKT8+cells from both adults and neonates suppressed PWM‐induced Ig secretion, but required the collaboration with cells within the OKT4+population. The suppressor activity in the PWM system was not IFN‐γ‐mediated.Thus, suppressor functions for EBV‐vs. PWM‐induced Ig synthesis were mediated through different pathways. There was no evidence of a unique suppressor system in the newborn. In the neonate, suppressor T cell activities are developed before T helper functions, a circumstance for which there