A study of interference between strains of tomato spotted wilt virus in tomato plants has revealed that the mild strain C1 protects plants against the severe strain A, whether this is inoculated before or after systemic symptoms of the mild strain appear (10 and 20 days respectively). Similarly, the mild strain E protects plants against the severe strain B. Quantitative detn. of yields of fruit and bush showed that a significant reduction in yield of both was effected by the severe strain when this was inoculated to plants already infected with the mild strain, although these "protected plants" yielded significantly more fruit and bush than the controls inoculated with the severe strain alone. When mixtures of strains were inoculated together in the same inoculum the same protective effects were observed and in no case did the severest symptoms appear. It is pointed out that previous theories to explain the phenomenon of protection do not satisfy the facts here described. A new theory postulates a transfer of character determinants between virus particles at some stage while they are multiplying in the cells they mixedly infect.