Changes in neutrophil response to N-formyl-methionyl-leucyl-phenylalanine (FMLP) and the phenotype of peripheral blood mononuclear cells were studied in monkeys after oral challenge with Shigellae. Monkeys were first challenged with S. dysenteriae 1 which caused shigellosis in some of the monkeys. After recovery, the monkeys were rechallenged with S. flexneri 2a. No difference in sensitivity was observed in the monkeys during shigellosis caused by either S. dysenteriae 1 or S. flexneri 2a. The optimal dose of FMLP for neutrophil polarization, a measure of early cell activation, in normal healthy monkeys was 10(-7) M when 67% of the neutrophils were polarized. Neutrophils from monkeys ill with shigellosis required higher doses of FMLP (10(-6) and 5 x 10(-7) M) for maximum polarization. As the monkeys recovered, a gradual decrease in the doses of FMLP for optimal neutrophil polarization was also observed. The percentage of CD2-positive T lymphocytes, the earliest marker for T lymphocytes in the peripheral blood, decreased when the monkeys developed shigellosis and returned to normal levels as the monkeys improved. However, there was no change in the percentage of CD20-positive peripheral blood B lymphocytes.