Abstract: Pancreatic exocrine function is determined by a breath test method wherein a radioactive labelled ester such as cholesterol octanoate is ingested in the body and is hydrolyzed by pancreatic enzymes and oxidized to labelled Co.sub.2 which is expired in the breath. The rate of appearance of labelled Co.sub.2 in the breadth is a function of the rate of hydrolysis by pancreatic enzymes which in turn reflects pancreatic exocrine function. Before ingestion, the ester is mixed with a carrier substrate such as a triglyceride to form a lipid matrix containing the ester and the lipid matrix is suspended in aqueous solution. To insure that low levels of expired labelled Co.sub.2 is due to pancreatic exocrine insufficiency, the ester is ingested simultaneously with a differentially labelled unesterfied carboxylic acid or the test method is repeated with an added step of simultaneously ingesting the ester with exogenously provided pancreatic enzymes.