The primary aim of this study was to assess the tolerability and immunogenicity of a new Haemophilus influenzae type b (Hib)/AlPO4 (CHIRON, SpA) vaccine, in two-month-old healthy infants. Twenty-three subjects were enrolled and administered the new Hib vaccine containing AlPO4 adjuvant at two, three and four months of age concomitantly with diphtheria-pertussis-tetanus (DPT) and hepatitis B vaccines according to the local program. Children were observed for 30 minutes after each immunization for any immediate local and systemic reactions. An active surveillance for side effects was performed on the 2(nd) and 7(th) days following each immunization by telephone. Families also filled out diaries for the first seven days. From the 2(nd) day to the next immunization only data about adverse events necessitating a physician's visit or about serious adverse events were collected. Blood samples were obtained before the first immunization and one month after the third dose for evaluation of anti-polyribosylribitol phosphate (PRP) antibody response. Local reactions at the Hib site were mild and less frequent compared to those observed at the DPT site. Systemic reactions noted after the three immunizations were fever in 70 percent, irritability in 48 percent, persistent crying in 26 percent, change In eating habits in 22 percent, diarrhea in 17 percent, sleepiness in 17 percent, vomiting in 9 percent, and unusual crying in 4 percent of the cases. There was no serious adverse event. One hundred percent and 95 percent of children achieved an anti-PRP antibody response over 0.15 mu g/ml and 1.0 mu g/ml, respectively. The geometric mean titer was 15 mu g/ml and the geometric mean ratio 84. It was concluded that the new (Hib)/AlPO4 vaccine is safe and well tolerated, and induced a good PRP antibody response in healthy two-month-old infants.