Abnormal levels of serum lactic dehydrogenase-1 (LD-1) activity have been observed in 81% (34/42) of patients with stage III germ cell malignancy of the testis. The criteria for evaluation the electrophoretic isoenzyme patterns of these patients were, as follows: For criterion 1 elevations the LD-1 value in absolute units was greater than 52.0 U/I with the LD-1/Total LD ratio greater than 37.2%. Criterion 2 elevations had absolute values of LD-1 less than 52.0 U/I, but the LD-1/Total LD ratio was greater than 37.2%. For criterion 3 elevations of LD-1, the absolute value was greater than 52 U/I and the LD-1/Total LD ratio was less than 37.2% activity with the LD-5/LD-1 ratio less than 0.5; or when the LD-5/LD-1 ratio was greater than 0.5 but the LD-1 is equal to or greater than the LD-2. The frequency of LD-1 elevation correlated well with the extent of the disease (stage II-B-1 and 2, 50%; stage III-B-3, 86%; stage III-B-4, 91%; stage III-B-5, 93%). LD-1 elevation occurred in groups I, II, IV and V histopathologic cell types (Dixon and Moore Classification) and there did not appear to be any correlation between the histologic cell type and the frequency of elevation of LD-1. Interpretation of LD-1 activity only on the basis of its relative ratio to the total LD value (criterion 1 and 2) identified a total of 28 patients (67%). A criterion 3 elevation was demonstrated in 6 (14%) additional patients. All patients with persistent elevations or recurrent elevations of LD-1 have shown progressive or recurrent disease and patients with no clinical evidence of disease have demonstrated normal LD-1 values. In those patients with elevated LD-1 activity, serial measurements of serum. LD-1 isoenzyme reflect the response of the patient to therapy.