SUMMARYA technique is described for titrating trachoma and inclusion blennorrhoea viruses by counting the inclusions formed in HeLa cell monolayers. The method compares favourably in accuracy with other techniques used for the assay of viruses and is more reliable than titration in the chick embryo yolk sac.

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SUMMARY: Four strains of Salmonella typhimurium avirulent for mice were found to be capable of acting as gene acceptors in transduction experiments. An attempt was made to transduce virulence to three strains, using a typical mouse-virulent strain of S. typhimurium as donor. Two of the strains which are adenine-dependent were successfully made virulent by transduction and simultaneously became adenineindependent. The other strain M 206, whilst possessing the typical antigenic structure of S. typhimurium and being non-exacting nutritionally, was not made virulent by transduction.Although most strains of a bacterial species behave similarly in their virulence for various animals, there are numerous examples of bacterial strains which are atypical in this respect. Bacterial virulence may be dependent on a number of factors and the absence of any one of these may result in loss of virulence. With bacterial strains which are capable of genetic recombination it may be possible to transfer the missing factor from a virulent to an avirulent strain, thus making the latter virulent. This was done in the case of the pneumococcus by Griffith (1928) ; rough avirulent non-capsulated strains were transformed into smooth virulent capsulated ones, confirming the importance of the capsule for virulence in this species. The present paper deals with attempts to transduce virulence within the species Salmonella typhimurium. METHODS