Total protein, alpha1-antitrypsin, alpha2-macroglobulin, amylase, methemalbumin, tryptic amidase activity, radioimmunoassayable elastase 2, and three lysosomal hydrolases were determined in the ascites fluid from patients with acute pancreatitis. In eight patients methemalbumin was detected in ascites and serum, supporting the diagnosis of hemorrhagic pancreatitis. Significant levels (4-45 microgram/ml) of tryptic amidase activity were detected in ascites samples from all patients. Evidence is presented which demonstrates that the tryptic amidase activity is due to alpha2-macroglobulin-bound trypsin. Pancreatic elastase 2, determined with a new sensitive and specific radioimmunoassay, ranged from 400 to 2100 ng/ml in serum and from 650 to 4460 ng/ml in ascites fluid. Substantial amounts of alpha2-macroglobulin-bound trypsin and elastase 2, entering the circulation from the peritoneal cavity, might be responsible for certain serious complications seen in acute pancreatitis. However, with the exception of serum calcium and methemalbumin and the ascites fluid methemalbumin and total protein, none of the biochemical parameters studied showed a distinct correlation with the patient's outcome.