In Vitro: 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 3,3'-Dihexyloxacarbocyanine iodide, there is a short lag period of 5 seconds. The lag period is followed by a rapid loss of fluorescence. Examination of 3,3'-Dihexyloxacarbocyanine iodide loaded neutrophils using fluorescence microscopy demonstrates that in resting neutrophils the brightest fluorescence is associated with long thin organelles which corresponds to the distribution of mitochondria.