Pertussigen (pertussis toxin (PT] is one of the most effective stimulators of IgE production in mice and rats. Employing flow microfluorimetric analysis (FMF), we showed that PT increases the percentage of blood and spleen lymphocytes with IgE on their surface. The percentage of IgE-bearing cells in the spleen of normal untreated C57Bl/10SCN mice of various ages varied from 2.2 to 12.2%, with an average value of 6.1 +/- 5.4%. In mice treated with 400 ng of PT and 1 mg of chicken egg albumin (EA), the percentage of these cells increased, 14 days after immunization, to an average value of 31.1 +/- 2.2%. Immunization of mice with PT alone increase the percentage of IgE-bearing cells only slightly (13.1 +/- 2.2% of the splenic lymphocytes) while injection of 1 mg of EA alone did not have any detectable action. As little as 6 ng of PT, when given simultaneously with 1 mg of EA, increased the percentage of IgE-bearing lymphocytes. A booster dose of 10 micrograms of EA given on Day 14 induced a further increase in the percentage of these cells even when as little as 0.039 ng of PT had been given at the time of initial immunization. PT was effective when given 4 days before or 5 days after EA. EA was effective when given 4 days before or 4 days after PT, but not 8 days after. The increase in IgE-bearing cells was mainly due to cytophilic binding of IgE to receptors for the epsilon chain of IgE (Fc epsilon) on the surface of lymphocytes rather than to a greater number of IgE-producing cells. This was shown by removing the IgE from Fc epsilon receptors by acid treatment which reduced the percentage of IgE-bearing cells to nearly normal values. The antibodies of IgE class with specificity to EA were increased dramatically, while antibodies with specificity to PT were not detected.