The interaction between 85Sr gamma radiation (average exposure approximately 220 R) and space flight factors (vibration, acceleration, weightlessness, etc.) on the number of flowers produced and on microspore division and development was studied in Biosatellite II. In the actual flight one package of 32 young Tradescantia clone 02 plants was irradiated during the two-day flight, and another package was used as an unirradiated flight control. Two similar packages, one of which was irradiated, were maintained as non-flight controls in a similar spacecraft at the launch site. Various postflight experiments were conducted in an attempt to attribute the effects observed in the orbited plants to specific flight factors. In these postflight ground tests plants were grown simultaneously in both the flight and non-flight spacecraft under conditions similar to those recorded during the actual flight. In the clinostat and additional vibration experiments individual packages rather than the whole spacecraft were used. After each treatment, open flowers were collected for at least 26 days. Some of the morphological and cytological effects observed in the flight material can be attributed to low gravity (weightlessness) and others to the internal environment of the spacecraft. Those effects attributed to weightlessness include (1) an increase in number of flowers produced in both orbited samples compared with their non-orbited controls and (2) various kinds of mitotic abnormalities associated with altered spindle behavior in microspores. Effects attributed to spacecraft environmental factors such as the increased concentration of ethylene measured in the flight vehicle include (1) an increase in the mortality rate of microspores, (2) an increase in the frequency of developmental abnormalities indicated by a change in the number and shape of the nuclei in young pollen, and (3) an increase in flower bud blasting which led to a change in the pattern of flower production.