The immunogenicity of the HbOC, a Haemophilus influenzae type b conjugate vaccine, was evaluated in a randomized clinical trial of Arab children resident in Tripoli. The HbOC vaccine was given as part of a three-dose series at 2, 3 and 4 months of age together with hepatitis B, OPV and DPT to 90 children. Anti-H. influenzae antibody levels were compared with 81 infants receiving hepatitis B, OPV and DPT but not the HbOC vaccine. The immunogenicity and safety of HbOC was as high as that observed in industrialised countries. There were no major complications, and fever and temporary local discomfort were observed in fewer than approximately 2% of the infants. Infants receiving the HbOC vaccine had an increase in Hib antibodies with only one dose. Geometric mean anti-Hib antibody levels were 0.41, 1.36 and 2.91 mg/ml after one, two and three doses. After two doses, all children had antibody levels above 0.20 mg/ml and the lowest antibody concentration was 0.80 mg/ml. Antibody levels in our children are similar to those observed in Europe and the USA and it is thus likely that HbOC will provide good clinical protection in this population. As most of the children develop antibody titres above or near 1 mg/ml, it is likely that they are protected even with two doses of the vaccine. The anti-Hib antibody levels observed are similar to those in studies from Europe where hepatitis B vaccine is not routinely given.