A quantitative relationship has been established between the number of particles, for example bacteriophages, counted in ultrathin sections of bacteria and the total number present in the whole bacterial cells. The factor F relating particles counted per section with the total number of these particles per entire bacterium could be arrived at by two methods, which proved to give results in close agreement. The first involves knowledge of the average volume of a bacterial section in proportion to the average volume of a whole bacterium; if the mean number of appearances of the same particle on consecutive sections is also known, F may then be calculated. The thickness of sections and, therefore, their volume, as well as the average number of times a single particle is sectioned could be learned by examination of serial sections. By counting the relative number of T2 phage particles which had been intersected once or twice, and relating this proportion to the known phage dimensions, the thickness of the sections was determined to be about 400 A.

The second measurement of F could be made in a particular case of late phage development where the number of particles per cell was countable or titratable directly in the bacterial lysate, this number being compared with the number seen in sections of the bacteria just before lysis.

The different sources of errors are discussed. The statistical error is under 20 per cent, while the systematic errors are higher and cannot yet be indicated precisely. After a very cautious estimation of the upper limits, we can state, however, that the counts made with this method are certainly reliable to well within a factor of two.

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