I n this study we further characterize the properties of the prostaglandin-producing suppressor cell. Overnight preincubation of peripheral blood mononuclear cells results in an increased response of the cells to phytohemagglutinin or Concanavalin A compared to the response of fresh cells. This increase in mitogen response with preincubation was similar in magnitude to the increase in mitogen response of fresh cells after the addition of indomethacin. The two manipulations were not additive; that is, after preincubation, indomethacin caused much less enhancement of mitogen stimulation of peripheral blood mononuclear cells (100 ± 12% increase before preincubation vs. 12 ± 6% after preincubation; mean±SEM, P < 0.001). Preincubated cells also lose sensitivity to inhibition by exogenous prostaglandin E₂. It requires the addition of 100- to > 1,000-fold more exogenous PGE₂ to produce comparable inhibition of phytohemagglutinin-stimulated preincubated cells than is required for inhibition of phytohemagglutinin-stimulated fresh cells.The enhancing effect of indomethacin increases with decreasing doses of phytohemagglutinin. Indomethacin causes a 1,059±134% increase in [³H]thymidine incorporation at the lowest dose of phytohemagglutinin (0.2 μg/ml), and a 4±3% increase at the highest dose (20 μg/ml). This increase in response to indomethacin with a lower dose of phytohemagglutinin is due to increased sensitivity to inhibition by PGE₂ at lower mitogen doses.The prostaglandin-producing suppressor cell assay and the short-lived suppressor cell assay measure over-lapping phenomena. The increased suppressive effect of the prostaglandin-producing suppressor at suboptimal mitogen dose must be taken into account in the interpretation of any study where the response to a range of mitogen doses is studied.