The ability of peripheral blood mononuclear cells (PBMC) from newborn infants, gestational age 24- 42 wk, to produce interferon-a (IFN-α) on the first day after birth was studied in vitro. Human amnion cells (WISH) coated with herpes simplex virus type I and fixed by glutaraldehyde were used as IFN-α inducers. Individual IFN-α producing cells (IPC) among PBMC were determined by an immunoplaque assay. The frequency of IPC was low in all premature (≤36 wk) infants (median 0.3 IPC/104PBMC, range 0.0-2.6), and significantly higher (median 2.0 IPC/104PBMC, range 0.0-16.4) in term infants (>37 wk). The frequencies were lower in both groups of infants than in adults (7.3 IPC/104PBMC, range 2.0- 23.7). When a conditioned medium from cultures of herpes simplex virus type I-stimulated PBMC from adults was added to the IFN induction cultures, the frequencies of IPC increased in PBMC from both preterm and term infants, and in the latter group did not differ significantly from adult levels. The median production of IFN-α per IPC was 1.1 U (range 0.0-2.8) in premature infants, 1.0 U (range 0.0-8.8) in term infants and 3.2 U (range 1.5-8.0) were decreased, a decline of IPC frequencies occurred. This decline was more marked and started at higher PBMC concentrations in infants than in adults, and was prevented by addition of conditioned medium from herpes simplex virus type I-stimulated cultures of PBMC from adults. The results suggest that PBMC of preterm infants on the first day after birth are deficient both with respect to the proportion of actual IPC and to accessory mechanisms necessary for a normal IFN-αresponse. In contrast, IPC frequencies in term infants approach levels of adults, but accessory functions may still be deficient.