Experiments in rats suffering from primary acute adjuvant inflammation showed independent changes in serum acute phase protein concentration and macroscopic paw inflammation during antiinflammatory treatment: soybean trypsin inhibitor and horse-radish peroxidase caused antiinflammatory effects but simultaneously produced increased alpha 2 macroglobulin levels. On the other hand, cycloheximide significantly inhibited the increase of alpha 2 macroglobulin concentration in adjuvant inflammation, however, it had no antiinflammatory effect. All forms of treatment caused even some change in protein plasma levels of healthy rats which probably relates to an activation of cells producing interleukin-1, interleukin-6, and/or hepatocyte stimulating factor which trigger the synthesis of acute phase proteins in the liver. In inflamed rats, the snake venom batroxobin caused a significant decrease in the fibrinogen level whereas the paw swelling remained completely unaffected. Therefore, it seems to be doubtful whether acute phase proteins essentially contribute to the modulation of acute inflammatory reaction in primary rat adjuvant inflammation.