Plasma prostaglandin (PGs) levels of normal individuals, cardiac and autoimmune disease patients were similar with the following mean and S.E.M. in ng/ml values: Normal, 45.4 +/- 6.4; cardiac, 48.3 +/- 4.2; and autoimmune, 52.4 +/- 4.8. On the other hand, cancer patients showed a significant increase in plasma PGS with a mean +/- S.E.M. of 102.0 +/- ng/ml as compared to the values of normal individuals, cardiac and autoimmune patients. It is suggested that, in part, the increase in PGs may be associated with the depressed state of the cell-mediated immunity in cancer patients.