The behavior of p-azophenylarsonate (Ars)-specific B cell clones during the primary T cell-dependent splenic response of A/J mice was investigated using an immunohistochemical approach. The earliest Ars-specific B cells were observed as isolated cells in the red pulp by day 3 after immunization with Ars-keyhole limpet hemocyanin,(KLH) and at day 6, large clusters of Ars-specific B cells were first detected in germinal centers, which continued to be observed for an additional 8 to 15 days. Surprisingly, no Ars-specific B cell foci were observed in or near the CD4 T cell-rich periarteriolar lymphoid sheath (PALS) during the entire primary response. Nevertheless, A/J mice immunized with (4-hydroxy-3-nitrophenyl) acetyl-chicken gamma globulin (NP-CGG) or Ars-CGG mounted robust splenic (4-hydroxy-3-nitrophenyl) acetyl or CGG-specific PALS-associated focus reactions, respectively. In contrast, no Ars-specific PALS B cell foci were detected in A/J mice immunized with Ars-CGG. These data add to a growing body of evidence indicating that B cell proliferation and differentiation in CD4 T cell-rich microenvironments are not prerequisites for the GC reaction. Taken together with previous results obtained using other model Ags, the data suggest that the specificity of the B cell Ag receptor may strongly influence the lymphoid microenvironment in which a B cell clone first undergoes Ag-driven clonal expansion and differentiation.