The primary immune response to T cell-dependent Ags develops in two pathways. These comprise the extrafollicular pathway, in which foci of Ab-secreting cells develop, and the intrafollicular pathway that gives rise to germinal centers and affinity maturation. We have previously shown that de-aggregated (4-hydroxy-3-nitrophenyl) acetyl (NP) coupled to the protein carrier human serum albumin (HSA), (NP-HSA), injected 6 days after challenge immunization with aggregated NP-HSA, resulted in impaired development of NP-specific, higher-affinity cells. Studies presented here describe the cellular basis underlying this impairment of affinity maturation. Using multiparameter flow cytometry, we show that mice injected with soluble NP-HSA ("tolerant" mice) develop significantly fewer NP-binding IgG1+ B220+ cells of germinal center origin than do the control ("immune") mice. In addition, using immunohistology, we noted that the spleens of tolerant mice had a marked reduction in the number of germinal centers that contained lambda-bearing cells, these being characteristic of the NP response in C57BL/6 mice. Curiously, germinal centers in the spleens of tolerant mice had more than twice the volumes of those in the immune spleens. In contrast to its effect on the germinal center pathway, soluble Ag enhanced the extrafollicular pathway, reflected by the increased numbers of B cells secreting IgM and IgG1 Abs specific for NP, HSA, and undefined Ags. Thus, soluble NP-HSA given after challenge immunization can impede affinity maturation of NP-specific cells, but enhance the extrafollicular pathway. These results are discussed in the context of the known capacity of some persisting Ags, e.g., malaria parasites, to frustrate affinity maturation and memory cell generation.