We evaluated the anti-HBs titers in 20 babies born to HBsAg-positive mothers and receiving different programs designed to prevent hepatitis B. Passive prophylaxis by HBIG for 6 months assured detectable antibody levels in the same period. 3 months after the last injection we could detect antibodies in 5 out of 9 cases. In no case could we detect antibodies 6 months after the last dose. Active prophylaxis by hepatitis B vaccine was able to raise anti-HBs response in 8 out of 12 babies after two or three very small doses (1.25 μg of antigen). 2 of these babies were given the first dose at birth. Small doses of vaccine were immunogenic in the presence of passive antibodies.