H uman antibodies specific, for polyribosyl-ribitol-phosphate (PRP), the capsular polysaccharide of Hemophilus influenzae b, were studied using idiotypic analysis. Antisera were prepared against purified F(ab')2 anti-PRP from two unrelated adults, H.H. and P.T. After repeated absorption with IgG myeloma proteins and with PRP-absorbed normal human Ig and donor Ig, anti-idiotypic (anti-Id) sera were obtained that specifically reacted with anti-PRP antibodies. Anti-IdHH and anti-IdPT reciprocally crossreacted with H.H. and P.T. anti-PRP antibodies and F(ab')2 fragments, and also reacted with the serum anti-PRP antibodies from three additional adults unrelated to P.T. and H.H. Both anti-Id sera partially inhibited anti-PRP paratopes but not anti-tetanus toxoid paratopes. PRP did not inhibit anti-Id recognition of shared or crossreactive idiotypic (CRI) determinants. Naturally occurring and PRP immunization-induced anti-PRP antibodies expressed CRI. While CRI titer increased after immunization, the increase was usually less than the rise in total anti-PRP antibody. Quantitative differences in CRI expression were also apparent between natural and immunization-induced H.H. and P.T. anti-PRP antibodies as shown by their differential inhibitability by anti-Id. Our data demonstrate that anti-PRP antibodies from five unrelated adults express CRI determinants that are probably distant from the PRP combining site. Naturally occurring and immunization-induced anti-PRP antibodies share CRI and therefore appear to be clonally related, although immunization apparently induces the expression CRI-negative antibodies as well. These results, taken with previous studies showing restricted and identical anti-PRP isoelectric focusing spectrotypes in unrelated adults, suggest that some PRP-specific V domains are structurally conserved and probably germ-line encoded.