A number of components isolated from styles of P. avium cv. Napoleon (S 3 S 4) have been tested for their capacity to influence in vitro growth of pollen tubes from fresh and stored pollen (cv. Napoleon (S 3 S 4)). An antigenic glycoprotein (Antigen S) is a potent inhibitor of in-vitro pollen tube growth, causing a 65% reduction in tube length at a concentration of 20 μg/ml. None of the other style components were effective inhibitors of pollen tube growth; neither were proteins of animal origin such as histone, serum albumin, cytochrome C, and the glycoproteins ovalbumin and thyroglobulin, effective inhibitors.