Meningococcal A and C but not B capsular polysaccharides stimulated a low level primary antibody response, predominantly IgM, and no secondary response in 21-day-old CBA/A mice. However, in 56-day-old mice a higher proportion of IgG antibody and a secondary response were produced. When the polysaccharides were injected in conjunction with rDNA derived human interleukin 2 (IL-2) the IgG antibody responses were increased in both age groups and memory cells were primed in the younger mice. IL-2 increased significantly the IgG antibody response to conjugates of A and C polysaccharides with diphtheria mutant protein but exerted a minimal effect on the IgG response to B polysaccharide complexed with aluminium hydroxide and outer membrane proteins. The stimulatory effect of IL-2 on the antibody responses to the polysaccharide antigens was not mediated by T-cells as similar results were obtained in athymic (nu/nu) and thymocompetent (nu/+) mice. However, the response to the A and C oligosaccharide conjugates was T-cell dependent and occurred only in the heterozygotes. In this case the adjuvant effect of IL-2 was seen only in the response to the C polysaccharide conjugate and was transferable with T-lymphocytes from primed animals.