A study on natural immunity to Haemophilus influenzae type b (Hib) was carried out in the province of Siena on 474 subjects ranging in age from 3 days to 70 years. The titration of antibody to capsular polysaccharide (PRP) was performed by the radioantigen-binding assay (RABA) method. A total of 66.67% of the population studied presented an antibody level considered to be protective (greater than or equal to 0.15 microgram ml-1). Seropositivity was 5.7% in the 5-7 month age group and 29.09% in the 8-17 month age group. This rose progressively in successive age groups reaching 79.54% between 4 and 6 years old and a value greater than 90% after 7 years old. From 3 to 17 months even the geometric mean of antibodies to PRP was below the protective limit. Our data indicate that, even in Italy, the majority of the infant population is not protected against H. influenzae, and therefore that vaccination should also be introduced in this country.