Four hundred ninety eight children of 15 months to 5 years of age were immunized with 0.5 ml of b-CAPSATM I Haemophilus influenzae type b polysaccharide vaccine (Hib). Every other child received the vaccine subcutaneously (S.C.) and the alternates received the vaccine intramuscularly (I.M.). Two hundred and two (81.1%) children in the S.C. group and 198 (79.5%) children in the I.M. group returned for follow-up visits 48-72 hours after the immunization and had the adverse reactions documented. The rest were contacted by telephone. After matching for sex and age, we had 194 pairs (103 males and 91 females) in each group with a mean age difference between the pairs of 0.4 month. Based on paired comparisons, pain, manifested as crying, at the time of inoculation was more frequent with intramuscular administration (p less than 0.01). Tenderness at the injection site occurred in 4 children in the I.M. group but none in the S.C. group and was of borderline statistical significance (p = 0.06). The occurrence of other adverse reactions was not significantly different between the two groups. The findings of this study suggest that subcutaneous injection is the method of choice for Hib vaccine administration.