The levels of 5'-nucleotide phosphodiesterase isoenzymes (5'-NPD; EC 3.1.4.1) in sera of 54 healthy donors and 201 inpatients were measured. Isozymes were separated electrophoretically and designated as 5'-NPD-0, -I, -II, -III, -IV and -V in the reverse order of their electrophoretic mobility. In healthy donors all isozymes except 5'-NPD-V were detectable. In pathological sera isozymes 0 to V were elevated in 26.0%, 20.5%, 14.0%, 30.5%, 7.0% and 15.0% of the cases, respectively. Decreased values were found in 6-7%, with the exception of 5'-NPD-IV showing decreased activities in 23.5% of the patients. This average distribution pattern was found in many disorders. However, in diseases of the liver and the pancreas a remarkable accumulation of cases with elevated levels of all isozymes, except 5'-NPD-IV, was observed. All isozymes, except 5'-NPD-IV, showed many significant correlations with other laboratory parameters indicating liver disease. Isozyme IV was not related to these parameters but exhibited a strong correlation with serum albumin. 5'-NPD-II was unproportionally often increased in patients with liver cirrhosis and was the only isozyme with on the average higher levels in women than in men.