Abstract Total lactate dehydrogenase (LDH) activity, LDH isoenzyme distribution, and the degree of substrate (pyruvate) inhibition were studied in alveolar macrophages (AM) and peritoneal macrophages (PM). Total LDH activity per milligram of protein is significantly greater in the PM (1.52 ± 0.74 S.D.) than in the AM (0.51 ± 0.10). The distribution of M and H units is substantially different with the relatively “aerobic” AM possessing 89 per cent M-LDH and the relatively anaerobic PM possessing 64 per cent of M-LDH, respectively. The “in vitro” substrate (pyruvate) characteristics parallel the percentage distribution of isoenzymes rather than the absolute M and H activities per milligram of protein. These studies suggest that M to H ratios are not a simple function of ambient O 2 tension or glycolytic capacity. The results are consistent with a difference in cell origin of the 2 cell types. Since PM clearly originate from circulating monocytes, the data support the possibility that AM may develop from local cells within the lung.