The results of previous studies on the proliferative response of resting cord blood mononuclear cells (CBMC) of term neonates to human recombinant interleukin 2 (hrIL-2) are contrasting. Some authors have reported a good and others a poor response. In our study we have obtained a significant reactivity, compared with the response of resting peripheral blood mononuclear cells (PBMC) of adult subjects, of resting CBMC to varying quantities of hrIL-2 in the absence of any known mitogenic or antigenic stimuli. Most responding cells was CD3 positive. Both CD4-positive and CD8-positive cells responded. However, we observed a more marked increase of the percentage of the CD4-positive T cells and a clear reduction of the percentage of the CD21-positive cells (B-lymphocytes) testing CBMC rather than PBMC of the adult subjects. The percentage of the NK cells was reduced in both the categories of subjects. Moreover, we have examined the reactivity to hrIL-2 of CBMC of preterm neonates. The results showed that this response is low. The peculiar level and kinetic of CBMC proliferative response to hrIL-2 are discussed.