We report the application of calorimetric method for evaluating the metabolic response of human granulocytes either from type I-insulin-dependent diabetes mellitus (IDDM) or from type II-noninsulin-dependent diabetes mellitus (NIDDM) in comparison to cells of healthy donors. Granulocytes either from patients (type I or type II diabetics patients) or from healthy subjects were exposed in a calorimeter to 4-β-phorbol-12,13-dibutyrate (PDB) or Zymosan opsonized particle (ZC3b). The total heat released by both PDB-activated or phagocyting granulocytes from type I-IDDM patients was significantly higher than that produced by granulocytes either from type II-NIDDM patients or from healthy subjects (p<0.05). The heat production by granulocyte from healthy subjects and from type II-NIDDM patients were similar (p>0.05). These results demonstrate, for the first time, the discrimination between human granulocytes from type I-IDDM and type II-NIDDM patients by calorimetry. Thus, we suggest that due to its high sensitivity, calorimetry may be used as an auxiliary method for discriminating type I-IDDM from type II-NIDDM patients based on granulocyte reactivity.