A quantitative immunoenzymatic assay has been developed for alpha-fetoprotein which is sensitive and specific. Seventy-eight percent of United States hepatoma patients have detectable serum alpha-fetoprotein elevations over 50 ng/ml, whereas only 2 of 93 other gastrointestinal tumors were positive. Thirteen percent of patients with acute viral hepatitis, 44% of patients with massive hepatic necrosis, and 23% of patients with chronic active hepatitis had measurable serum alpha-fetoprotein concentrations. However, patients with non-viral acute or chronic liver disease were largely alpha-fetoprotein negative and alpha-fetoprotein was undetectable on multiple postoperative samples from 6 patients after hepatic lobectomy in the rapidly regenerating phase. Therefore, alpha-fetoprotein elevations in nonmalignant liver diseases are not due solely to hepatic regeneration but appear to be related to viral injury. The immunoenzymatic assay does not require purified antigen or radioisotope equipment and can detect and quantitate clinically significant alpha-fetoprotein levels greater than 50 ng/ml.