We investigated the antigen-specific sensitization of basophils in the peripheral circulation of guinea pigs using an in vitro histamine release assay. Animals were primed for cutaneous basophil hypersensitivity (CBH) by intradermal injection of either keyhole limpet hemocyanin (KLH) or ovalbumin (OA) in saline. Basophils isolated from these animals 7 days after sensitization did not release histamine to the specific antigen in vitro. On day 9, however, basophils did release histamine when challenged with the antigen. These results were in contrast to 24-hr skin tests that showed a typical CBH profile both on day 7 and day 9. When animals were immunized via the footpad by emulsification of antigen in incomplete Freund's adjuvant, basophil sensitization (i.e., histamine release) either occurred earlier (day 7, KLH) or at lower antigen concentrations (OA). Thus, dissociation of in vivo CBH and in vitro basophil sensitization was demonstrated. The lack of correlation was only seen early after immunization and depended on the route of immunization used. An additional finding was a transient blood basophilia that also was dependent on the route of immunization, the antigen used, and the time post-sensitization. Our results are consistent with the hypothesis that at early times post-immunization, CBH is mediated by T cells and not via homocytotropic antibodies, but that at later times such antibodies may also play a role in the elicitation of CBH reactions.