We evaluated three different methods for measuring lactate dehydrogenase (EC 1.1.1.27) isoenzyme LD-1 activity--agarose gel electrophoresis and two immunoassays, Isomune-LD (Roche) and LD-1 Immuno (Seragen)--in patients' samples for which measurement of creatine kinase-MB was ordered. Regression analyses of the comparisons gave the following results: LD-1 (%) from Isomune-LD (y) vs electrophoresis (x) (n = 51), y = 1.05x + 1.99, r = 0.92; LD-1 (%) from LD-1 Immuno (y) vs electrophoresis (n = 27), y = 1.05x + 3.94, r = 0.88; LD-1 (%) from LD-1 Immuno (y) vs Isomune-LD (x) (n = 41), y = 1.06x + 0.48, r = 0.95. Comparison by Student's paired t-test yielded significant differences between the mean values by electrophoresis and both Isomune-LD (P less than 0.005) and LD-1 Immuno (P less than 0.001), but no significant difference between the two immunoassays (P greater than 0.2). Analyzing these results by the overlap index, we conclude that electrophoresis shows the best clinical correlation followed, in order, by the Isomune-LD and the LD-1 Immuno methods. Both immunoassays are simpler and more rapid than electrophoresis, but in our hands the Isomune-LD method demonstrated greater precision and better correlation with electrophoretic values.