Different germination rates were found for pollen of P. domestica regarding both germination media and incubation temperature and duration. To arrive at comparable results, germination of pollen in vitro in 10-20% sucrose or glucose solution should be tested at 22.degree. C (.+-. 2.degree. C) for 24 h. The rates of pollen germination of the 12 varieties tested were between 2%-64%. High rates were observed for pollen of Reine Claude d'Althans, The Czar, Hauszwetsche (German prune), Italian Prune, Kirke's Plum, Opal, Stanley, Wangenheim's Early Prune. Pollen of Luetzelaschser Early Prune and Mirabelle de Nancy showed mediocre germination, and the rates were below 15% for pollen of Ersinger Early Prune and Zimmer's Early Prune. The latter varieties, therefore, are considered unsuitable as pollinators. Germination rates in vitro and in vivo of some varieties varied considerably from 1 year to the other. The in vivo rates were higher than those observed in vitro; they ranged from 32-96%. Pollen of any one variety reacted differently on stigmas of different varieties. Different environmental conditions (open vs. greenhouse) yielded different results. Comparability of in vivo and in vitro is discussed on the basis of the methodological problems.