Abstract The memory response to PC-KLH is dominated by two antibody populations differing in fine specificity. Group I antibodies show affinity for both phosphocholine (PC) and p-nitrophenyl phosphocholine (NPPC). Group II antibodies exhibit significant affinity only for NPPC. Here, we describe the binding site charactersitics of Group II antibodies and show that in recognizing NPPC these antibodies have a common requirement for the phenyl moiety, a negatively charged phosphate, and the trimethyl structure of the choline. However, Group II antibodies were found to differ in their requirement for the positively charged nitrogen of choline and thus could be divided into two subgroups. In contrast to Group II-A, Group II-B antibodies recognize not only NPPC but also its analog p-nitrophenyl-3,3-dimethyl butyl phosphate (NPDBP), which differs from NPPC by substituting a carbon for the positively charged nitrogen of the choline moiety. These results suggest that Group II-B antibodies do not require the positive charge in order to bind, although the binding constant, Ka, was increased when the nitrogen was present. Furthermore, heterogeneity within Group II antibodies was characterized by differences in binding to clinitrophenyl phosphocholine which has an additional phenyl ring and aminophenyl phosphocholine which has an amino group in place of the nitro group of NPPC. The results indicate that diversity in the memory response to PC-KLH is reflected in the Group II antigen-binding phenotype by antibodies which differ appreciably in their recognition of various structural aspects of the hapten.