Antibody responses to 14-valent pneumococcal capsular polysaccharide vaccine were measured by Farr-type radioimmunoassay in children younger than 7 years of age. On the basis of immunogenicity in young children individual pneumococcal polysaccharides could be identified as uniformly good, strongly age-dependent or uniformly poor immunogens. Pneumococcal types 6A and 23F, which frequently cause pneumococcal infections in small children, were the poorest immunogens in this age group. The children younger than 2 years of age responded very poorly also to types 19F and 18C whereas older children had good antibody responses to these types. The results support the current view that present pneumococcal polysaccharide vaccines are not beneficial in children younger than 2 years of age and stress the importance of attempts to improve their immunogenicity.