Prostaglandin E1, a mediator of inflammation, was investigated for its effects on serum acute phase proteins, alpha 2 macroglobulin (alpha 2M). Induction of carrageenin inflammation in rats caused an elevation of alpha 2M to a maximum level (100%) at 1 day. Similarly, administration of PGE1 (1 mg/kg) was found to increase serum alpha 2M levels in normal rats. On the other hand, sc injection of PGE1 into inflamed rats significantly reduced the alpha 2M in serum as well as edema. In vitro studies with liver slices showed increasing rates of incorporation of [14C]leucine into alpha 2M with the addition of PGE1 to the medium. It was followed by the secretion of alpha 2M-bound radioactivity into media. But addition of higher doses (greater than 100 ng/ml) of PGE1 resulted in the suppression of incorporation and secretion of alpha 2M-bound radioactivity. Incubation of inflamed liver slices with PGE1, however, showed only decreased incorporation and secretion of alpha 2M-bound radioactivity. These results indicate that (a) primary prostaglandins, like PGE1, generated during inflammation may be responsible for the increase of alpha 2M in serum, and (b) PGE1 enhanced the synthesis of alpha 2M in liver and its secretion into the medium, so the anti-inflammatory drugs which decrease levels of PGs are likely to alter alpha 2M levels.