Abstract
The ts lesion of the fowl plague virus (FPV) mutants ts 18 and ts 236 has been located in RNA segment 2 (Ptra gene, corresponding to P3). After double-infection with these mutants and ts 90 or ts 93, which also carry a ts lesion in segment 2, plaques were formed at the non-permissive temperature (40 °C). These plaques cannot be passaged at 40 °C and exhibit a morphology which differs from those formed by the wild-type virus. The yield of infectious particles after double-infection shows a non-linear correlation between the plaque number and dilution, indicating that at least two particles are needed for infection of a cell. All experimental evidence points to an intracistronic complementation within the P3 protein.