Cord blood lymphocytes were stimulated with phytohaemagglutinin (PHA) to produce interleukin-2 (IL-2) and immune interferon (IFN-gamma). On PHA stimulation, cord blood lymphocytes produced efficiently IL-2 as much as adult ones. Antiviral activity generated on PHA stimulation was shown to consist mainly of IFN-gamma as assessed by the sensitivity to pH 2.0 treatment and neutralization with anti-human IFN-gamma antibody. In contrast to IL-2 production, cord blood lymphocytes released extremely low levels of IFN-gamma following PHA stimulation. The producing ability of IFN-gamma by lymphocytes on PHA stimulation gradually increased with child growth, but was significantly low at 1-2 years of age as compared with adult controls. Around 3 years of age or later, the producing ability of IFN-gamma by lymphocytes on PHA stimulation attained levels comparable to those of adult cells. These results suggested that IL-2 producing ability of lymphocytes appeared to be at a mature stage at birth, whereas lymphocytes in the early human life might be relatively deficient in their ability to produce IFN-gamma.