Crosses between various strains of Drosophila melanogaster lead in some cases to a quite typical female sterility which involves non-mendelian hereditary factors. On the basis of the fertility of F1 females, strains can be divided into three classes: Inducer, Reactive and Neutral. Females showing various degrees of sterility arise when reactive strain females are crossed to inducer males. The degree of sterility depends on the particular reactive and inducer strains used in the cross. Quantitative variations in the efficiency of inducer and reactive strains to produce sterile F1 females are studied in the present paper. The results indicate that the order which can be established within a set of reactive strains for this efficiency is largely independent of the inducer strain which is chosen for the cross.