In order to ascertain the relationship, if any, between cross-sterility in hybrid plums and cold weather, the germination and growth capacity of pollen of different varieties at low temperatures was investigated. Most of the Minnesota hybrid plums produce abundant pollen but the viability is low and few can be regarded as dependable pollinators. Native plum varieties produced only fair pollen. As regards temperature, at 10 degrees C. the germination of all varieties tested was lower than at higher temperatures. The rate of pollen tube growth was markedly affected by temperature, and at 10 degrees C., the lowest temperature at which plum pollen germinated within 24 hours the tubes were very short. At 13 degrees C. growth was much more rapid and at room temperature long tubes were obtained in 24 hours.Though the relation of the age of pollen to germination and pollen tube growth was not determined by exact counts, in general the viability percentage decreased as the age increased. Age also affected the vigour of pollen tube growth.