Spleen cells from newborn mice are immunologically non-reactive and do not respond by proliferation to T- or B-cell mitogens. However, they synthesize significant levels of mRNA for interleukin-1 alpha (IL-1 alpha) and tumour necrosis factor-alpha (TNF-alpha) after mitogen stimulation. Since these two cytokines mediate many protective activities in the body, they may be important in ensuring the survival of immunologically immature newborns.