Plasma membrane fluidity of polymorphonuclear leukocytes was investigated in 28 patients with insulin dependent diabetes mellitus and 30 healthy controls. Membrane fluidity was measured by steady-state fluorescence anisotropy of 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) incorporated into the plasma membrane. The fluorescence anisotropy values in resting (unstimulated) polymorphonuclear leukocytes from diabetic subjects were significantly higher than those of controls (0.318 +/- 0.003 vs 0.287 +/- 0.003, P less than 0.001). The addition of the respiratory burst stimulus phorbol myristate acetate induced a stable increase in fluorescence anisotropy values in both groups. Fluorescence anisotropy values of stimulated polymorphonuclear leukocytes from the diabetic and control groups were not significantly different (P greater than 0.05). These data demonstrate a decrease in plasma membrane fluidity of resting polymorphonuclear leukocytes obtained from diabetic subjects. This finding could be in part explained by an increase in their basal respiratory burst activity.