The capacity of peripheral blood mononuclear cells (PBMC) to produce interleukin (IL) IL-1beta, IL-2, IL-3, IL-6, IL-10 and tumor necrosis factor-alpha (TNFalpha) was examined in term newborns with hyperbilirubinemia after 24 hours' exposure to phototherapy (wave length 425-475 nm). The results were compared with those from untreated neonates. Fifty newborns spontaneously delivered at term were included in the study. Blood samples were collected from 20 newborns before and 24 h after phototherapy. The control group consisted of 30 neonates examined on two consecutive days. PBMC isolated from blood samples were incubated in vitro for cytokine production. The concentration of cytokines in the supernatants was tested using ELISA kits (for IL-1beta, IL-6, IL-10 and TNFalpha), or by bioassays (for IL-2 and IL-3). Phototherapy caused a 70% increase in IL-2 secretion (123 +/- 27 vs 208 +/- 30 units/ml, P < 0.01) and 56% in IL-10 production (1.07 +/- 0.19 vs 1.67 +/- 0.33 ng/ml, P < 0. 03), whereas the spontaneous secretion of IL-1beta was reduced by 43% (13.7 +/- 2.3 vs 7.3 +/- 1.7 ng/ml, P < 0.02). In the control group the secretion of these cytokines was similar on the two consecutive days and did not differ significantly from secretion in the other group before phototherapy. On the other hand, lipopolysaccharide induced TNFalpha production was higher on the second day in the two groups of newborns irrespective of phototherapy (388 +/- 58 vs 683 +/- 88 pg/ml, P < 0.001, in the control group and 384 +/- 75 vs 588 +/- 91, P < 0.05, before and after phototherapy). The synthesis of IL-3 and IL-6 did not change significantly between the two days of the study. The results demonstrate that in addition to the well-known positive effect of phototherapy on the neonate serum bilirubin level, this treatment affects the function of the immune system in newborns via alterations in cytokine production.