A radioimmunoassay available for human aldolase A was developed for the direct quantification of aldolase A in human serum and tissues. The method was a double antibody technique using radio-iodinated purified aldolase A, chicken antibody to aldolase A, and rabbit antibody to chicken IgG. This radioimmunoassay was specific for the aldolase A subunit, with no cross-reactivity with human aldolase B subunit or human aldolase C subunit. Aldolase A was predominantly high in skeletal muscle, and relatively high in brain and cardiac muscle. Normal liver tissue contains only a small amount of aldolase A, whereas aldolase A predominates in liver cell carcinoma tissue. Aldolase A levels in the sera of normal subjects were 171 +/- 39 ng/ml (mean +/- 2SD). Aldolase A levels correlated closely with hemoglobin levels, so aldolase A levels were increased in the sera with hemolysis. Since skeletal muscle is the largest origin of aldolase A in any tissue, serum aldolase A levels were increased in patients with acute muscle injury due to abdominal operation and even in healthy subjects after hard exercise. Serum aldolase A levels in almost all of non-cancer patients of the digestive tract were less than 210 ng/ml. In contrast, about 80% of patients with cancer in the digestive tract showed increased serum aldolase A levels. Aldolase A levels were remarkably increased in the sera of cancer patients with distant metastasis. The CEA levels were increased only in 46% of the sera of patients with cancer in the digestive tract, whereas the aldolase A levels were increased in 86% of the patients. The AFP levels were higher than 100 ng/ml in 76% of the sera of 21 patients with liver cell carcinoma, whereas the aldolase A levels were markedly increased in 90% of them. From these results, it may be suggested that the determination of serum aldolase A by radioimmunoassay is a useful tool in the clinical diagnosis of cancer patients with cancer in the digestive tract.