Seventy-eight children who had received three very low doses (1 or 2 microg) of Merck, Sharp and Dohme (MSD) plasma-derived vaccine (PDV) in early infancy were followed to approximately four years of age. Of the 70 who had responded to the initial course of vaccine with measurable anti-HBs, levels had fallen to below 10 mlU/ml in 38% of subjects given 1 microg doses and in 17% of those who had been given 2 microg doses. None of the children were positive for anti-HBc. Two weeks after 2 microg dose of MSD recombinant DNA (rDNA) vaccine all subjects had more than 10 mlU/ml of anti-HBs, with 90% exceeding 1,000 mlU/ml. A response to hepatitis B vaccine in infancy is followed by an effective immunological memory for several years, even if anti-HBs falls to low levels. The rDNA hepatitis B vaccine (MSD) in 2 microg doses is an effective booster following a primary course of plasma derived vaccine.