Cytokines IL-1-beta, IL-2, and TNF alpha were detected in occasional cells within portal inflammatory infiltrates beginning 3 weeks after oral inoculation of monkeys with HAV. The number of cells secreting those cytokines did not increase, and they were not of importance in the pathogenesis. Production of cytokines IL-6 and IL-4 by T lymphocytes infiltrating portal areas started 4 weeks after inoculation, stimulating local expansion of B cells, probably secreting antibodies to HAV. IL-6 and IL-4 may also stimulate cytotoxic activity of a few CD8+ lymphocytes.