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A modification of the Deitch "standard" naphthol yellow S technique is described and validated for use in deoxyribonucleic acid and protein determinations in drosophila larval material. By the use of this technique in conjunction with cytophotometry relative amounts of protein have been determined in 436 salivary gland nuclei from 37 individuals of wild-type, lgl lethal, and experimentally treated larvae of D. melanogaster and in 294 nuclei from 44 individuals by photometric interferometry. Altogether, 8 comparisons were made by comparing the ratio between two kinds of material obtained by naphthol yellow S cytophotometry with that obtained on the same kind of material by photometric interferometry. In 6 of the first 7 comparisons the cytophotometric ratio could be reconciled with the interferometric ratio by an adjustment well within statistical probability. In the seventh comparison, the ratios could not be reconciled by any adjustment within statistical probability but could be so reconciled when repeated on additional material of the same kind. Inconsistency of naphthol yellow results between the original and repeat comparisons indicated the probability of cytophotometric error due to nonproportionality of staining in the material used. Extensive additional tests revealed no further instance of inconsistency and implicated the material rather than the method in the one aberrancy discovered. It is concluded that the cytophotometric and interferometric data are in mutual support and indicate that either technique is a reliable method for protein determination; but that the naphthol yellow S technique may result in an occasional aberrancy in drosophila material that must be guarded against. Advantages and disadvantages of each technique are discussed.

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