The authors investigated C1-inhibitor (C1-INH) plasma levels in blood collected from mothers during delivery: mothers with a normal pregnancy and a group of mothers who displayed during the perinatal period direct or indirect signs of infectious disease. Both groups, as compared with a group of healthy donors, had low C1-INH levels. The neonates were divided into two groups (normal children and children with perinatal risk of infection) with sub-groups of mature full-term neonates (38-41 weeks of gestation) and premature infants (29-36 weeks of gestation). In the group of normal infants the neonates had on average somewhat lower C1-INH levels, as compared with healthy donors, in premature neonates of this group the lowest mean C1-INH level was recorded. In this group the authors observed a correlation between the inhibitor level and the gestation period. The results of C1-INH assessment in the group of neonates with a perinatal risk of infection were different. In premature neonates a higher average C1-INH level was observed than in mature neonates and the relationship between the C1-INH level and the gestation period was a linear negative regression. The postnatal increase of C1-INH levels on the 1st to 5th day was more rapid in premature neonates of both groups.