The number of cross-pollen tubes in the styles of self-incompatible kale plants was reduced by 46–94% when self-pollen was applied to the stigma immediately before the cross-pollen. Similar reductions were found when the cross-pollen was applied at time intervals of 2–16 h after the self-pollen, but little or no reduction occurred when the time interval was 24 h or 32 h. No reductions were found when cross-pollen was applied before self-pollen nor when dead cross-pollen was applied before the live cross-pollen. Application of onion pollen before cross-pollen gave a mean reduction of 69%. The results are discussed in the context of current views on the mechanism of self-incompatibility in Brassica, and their practical implications for the breeding of F1 hybrid brassicas.