The experiments described were undertaken to determine which cells of guinea pigs immunized by different ocular routes produce the IgGl and IgG2 antibodies detected in the serum.Guinea pigs were immunized intravitreally, topically, or by intravitreal immunization followed by topical conjunctival challenge. An indirect plaque assay was used to detect antibody producing cells in the cervical lymph nodes and ocular tissues. Passive hemagglutination, passive cutaneous anaphylaxis and ELISA assays were used to detect serum antibody.Both the topical and intravitreal methods initiated a primary antibody response. The guinea pigs developed IgGl and IgG2 serum antibodies, but IgE and IgA antibodies were not detected. IgGl and IgG2 plaque forming cells (PFC) were found in the lymph node and uveal tissues of the intravitreally immunized guinea pigs, and in the lymph node and conjunctival tissues of the topically immunized animals. IgA plaque forming cells were not detected in topically immunized animals.Ho antibody producing cells were found in intravitreally immunized guinea pigs sacrificed after the first conjunctival challenge (two months after sensitization). The highest numbers of lymph node and conjunctival PFC were found in the animals sacrificed three days after the second or third topical challenge. The numbers of IgGl antibody producing cells in this group of guinea pigs were usually higher than the numbers of IgG2 PFC. Serum antibody levels, undetectable before challenge, increased after the second challenge.We conclude that both lymph node and ocular cells produce antibody in guinea pigs immunized by ocular routes.