An anomalous zymogram of lactate dehydrogenase (LDH) in the serum from a patient with liver cirrhosis was reported. Agar-gel electrophoresis of serum showed an extra LDH band close to the anodic side of LDH5 and a wide band of LDH5. Gel filtration of patient's serum in Sephadex G-200 demonstrated an abnormal LDH fraction eluted between immunoglobulin G (IgG) and macroglobulin in addition to a normal LDH component. Chromatographically abnormal LDH was demonstrated on agar gel as extra and wide LDH5 bands and resembled closely human hepatic LDH in various physico-chemical properties such as inhibition by urea or substrate, stability against heat, and Michaelis-Menten's constant. Immunological analyses demonstrated that abnormal LDH could be in the state combined with IgG. Molecular weight of the complex estimated by gel filtration was approximately 300,000. Mixtures of the heated patient's serum with normal or patient's hepatic LDH showed abnormal LDH fraction by gel filtration, whereas abnormal fraction was not demonstrated when heated normal serum was mixed with normal or the patient's hepatic LDH. These results strongly suggest that the occurrence of anomalous LDH zymogram in patient's serum is due to a formation of LDH-IgG complex, which is based on the binding of essentially normal hepatic LDH and abnormal IgG.