This report describes hitherto unrecognized immunoenhancing properties of acetaminophen on cultured human lymphocytes. The addition of acetaminophen (2.5--300 micrograms/ml) significantly enhanced mitogen-induced blastogenesis. The stimulation of cells by Candida albicans, from individuals who were skin-test positive for antigen and two-way responses of mixed lymphocytes, were similarly augmented. Cells stimulated in the presence of acetaminophen were comparably enhanced in cultures lasting for 24--168 hr. Enhancement of stimulated cells was noted only if the drug was added during the first hours of culture. Exposure of cells to acetaminophen before mitogen stimulation did not enhance responses unless tha cells were simultaneously cultured with mitogen and the drug. Cells obtained from normal subjects before, during and after ingestion of therapeutic amounts of acetaminophen for 3 days, were cultured in a medium without acetaminophen, and did not display increased in vitro responses during or after drug consumption. These data indicated: (1) pharmacological concentrations of acetaminophen significantly enhanced lymphocyte responses to specific, non-specific, soluble and insoluble stimulation in vitro; and (2) enhancement by acetaminophen occurred only when the drug was in contact with cells during the early period of cell activation.