Neonatal lymphocytes include a subset of E-, OKT3-, OKT4-, OKT8+, HNK-1-, OKM1- cells displaying natural killer (NK) activity. More than 50% of these cells react with the B73.1 monoclonal antibody, moreover most of them bear the DR antigen, the receptor for Peanut agglutinin (PNA) and react with the OKT10 monoclonal antibody. This neonatal subset displays a higher NK activity than unseparated cord blood lymphocytes (CBL) and the present study shows that it is sensitive to boosting effect of IFN-B preincubation. When stimulated with T cell mitogens E-, OKT3-, OKT8+ CBL both produce Interleukin-2 (IL-2) and proliferate. Moreover this neonatal subset produces Interleukin-1 (IL-1) both spontaneously and in response to lipopolysaccharide (LPS). It has been previously suggested that these neonatal cells include a common precursor of NK and T cells. The present study further strengthen this hypothesis.