The authors investigated the reaction of four sour-cherry ('Debreceni botermo', 'Érdi botermo', 'Meteor korai', 'Újfehértó i fürtös') and three European plum cultivars ('Besztercei', 'Cacanska Lepotica', 'Stanley') to an auxin-synergistic preparation (Nevirol 60 WP). Spray applications were carried out at the 50% flowering stage of the trees. Application of the regulator increased the longevity/viability of the stigma and supported better pollination. The date of the end of flowering was deferred 2.2 days on average in the cases of sour-cherry cultivars, and 1.0-1.5 days on European plum cultivars. The treatment resulted in a 27-35% increase in fruit setting on sour-cherry, and 15-24% on plums. In the case of the sour-cherry cultivars the ratio of clusters with high numbers of fruits (3-4 fruit/bunch) was increased. In accordance, the yield was enlarged by about 22% in the case of sour-cherry cultivars and 15% on European plum cultivars. The increase of yield was not in close corellation to increases of fruit setting in all cases, because some fruits shrivelled or dropped as a consequence of oversetting in certain cases. Fruit quality parameters were also studied, including fruit weight, fruit diameter, soluble solids and sugar contant.