Infants born to HBsAg-positive mothers are at high risk of contracting perinatal hepatitis B infection. The prevention is based on active as well as passive immunoprophylaxis. We have used hepatitis vaccine in 18 newborns of as many HBsAg-positive mothers. Some haematologic and immunologic parameters are here reported. No alterations were observed as to liver function. Immunoglobulin values were normal for age. Auto-antibodies and rheumatoid factor were constantly absent. Immunecomplexes were present in the serum of some infants. The study of T cell subsets and of natural killer cells activity did not reveal any important changes, whereas minor modifications were present in polymorphonuclear leucocyte function. In all infants submitted to vaccination serum conversion was observed a with different antibody levels.