Introduction. Many viruses produce characteristic lesions, known as pocks or plaques, on the chorio-allantois of the developing chick embryo. The first to be successfully propagated on this membrane was fowl pox (1). Since then, Burnet and his collaborators (2, 3) have determined the potencies of suspensions of a number of viruses such as vaccinia (4), herpes simplex (5), influenza (6), ectromelia (7), myxoma (8), laryngotracheitis (9), and fowl pox (10), by estimating the average number of pocks produced at several serial dilutions. Some independent workers have found the technique satisfactory (11), but others have failed to obtain consistent results (12), and the method has not been generally adopted for the titration of viruses outside Burnet's laboratory.In these Laboratories, various studies on fowl pox virus, for which reliable titration methods are necessary, are in progress. Since titration by application of virus suspensions to the comb or feather follicles of adult birds seemed unsuitable, it was decided to investigate the possibilities of the chorio-allantoic method.
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