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161also eliminated the lag period for sorbitol but not for lactose. The delayed reduction of NT corresponds to the absence of lactose fermentation, and apparently this phenomenon represents the period of bacterial adaptation. In the case of sorbitol, however, either adaptation or a change in the substrate by pH and long preincubation may account for absence of the lag period.C. Morphology. Formazan was seen in the cells as discrete granules without diffuse staining. In all cells and with each substrate used, these granules were seen in a subpolar location characteristic for this strain of E. coli and for several other strains examined but not reported here. Occasionally, smaller cells had a single granule near one end. Most cells, however, possessed one granule in each polar region, and the larger cells often had a third eccentric deposit in the midportion of the cell. As our experiments progressed, the granules slowly grew in size at approximately the same rate in aU cells. In E. coli K-12, Lederberg," using triphenyltetrazolium, observed a single large formazan granule near one pole of the cell.Attempts to stain killed bacteria with formazan in alcohol solution or by reducing NT chemically to formazan in the presence of dead bacteria were unsuccessful. This indicates that the sites of formazan localiza-°t ion are the sites of NT reduction by various dehydrogenase systems. Our studies suggest the presence of organoid polar areas of dehydrogenase activity in E. coli. These areas may be analogous to cytoplasmic organoid structures of plant and animal cells.Summary.(1) A large variety of organic substrates known to be utilized by the organism have been shown to reduce neotetrazolium to its colored, insoluble formazan in the presence of washed suspensions of actively metabolizing E. coli.(2) Direct examination of bacteria exposed to NT and substrate allows for the detection and rough estimation of the enzyme activity of the individual cell.(3) The lag period in lactose and sorbitol utilization is characterized by the absence of NT reduction.(4) The regular appearance of formazan granules in the polar regions of the bacterial ceIl suggest that these areas are organoid and represent the sites of dehydrogenases.Neotetrazolium, a new tetrazolium derivative, has been utilized in a series of investigations concerned with root tip growth and cytology. This redox indicator, slightly yellow in color, has several advantages over tetrazolium salts previously used in studies of cellular physiology, as indicated by Antopol, Glaubach, and Goldman.' The stock solution of neotetrazolium (abbreviated as NT) was 0.5 per cent in strength. NT in 0.85 per cent sodium chloride gave similar results to those employing NT in tap water. The latter solution was then used, since it could be applied directly to the jars containing the growing roots. Concentrations of NT ranging from stock strength to 1 part NT in 2 million parts water .were used, and in vivo as well as supravital (excised roots) effects • This paper, illustrated with lantern sl...