In mice, immunological tolerance to self is established in the perinatal period, when tolerance susceptibility to allogenic tissues is higher than in adults. We have now investigated whether this could result from developmental regulation of effector functions of T cells exposed to specific antigens, by studying the "natural" or T cell receptor-induced expression of several interleukin genes. We used qualitative and quantitative polymerase chain reaction methods to study interleukin (IL)-2, IL-4, IL-10 and interferon-gamma mRNA expression by splenic cells at different ages. The results show that newborn peripheral cells (up to day 7), in contrast to the T lymphocytes of adult mice, express high levels of IL-4 and interferon-gamma, and very low levels of IL-2 messenger spontaneously and upon specific T cell activation. This characteristic phenotype depends on intrinsic T cell properties, as it is not due to the newborn environment.