Paired serum and heparinized plasma samples were assayed simultaneously for lactate dehydrogenase (EC 1.1.1.27) isoenzyme 1 (LD1) activity by a commercially available immunochemical procedure. For all sera specimens tested, only LD1 activity was detected. For heparinized plasma, random discrepancies in LD1 activity were noted at normal (Group I), borderline (Group II), and increased (Group III) total LD activity. Incomplete precipitation of LD-M subunits was confirmed by electrophoresis and occurred in eight of 15, four of 13, and eight of 22 instances (total: 20/50, or 40%) with a mean difference of 13, 10.6, and 18.2 U/L (8.3, 4.0, and 4.4%) in Groups I, II, and III, respectively. We conclude that heparinized plasma is an unsuitable sample for the immunochemical determination of LD1.