The addition of cells to the cuvette containing 0.25 μM 3,3'-Dihexyloxacarbocyanine iodide (DiOCg(3)) leads to an increase in fluorescence, equilibration is rapid and is complete by 4 min. When f-met-leu-phe (10-7 M) is added to neutrophils preequilibrated with DiOCg(3), there is a short lag period of 5 seconds. The lag period is followed by a rapid loss of fluorescence. Examination of DiOCg(3) loaded neutrophils using fluorescence microscopy demonstrates that in resting neutrophils the brightest fluorescence is associated with long thin organelles that correspond to the distribution of mitochondria.