A simple medium is described which supports the germination of Brassica pollen in vitro. The method is completely reliable and the percentage germination achieved is comparable to that found on a compatible stigma. The pollen tubes produced attain lengths equal to those growing in the style and exhibit similar growth kinetics. The major difference between this medium and other less-reliable media is the addition of 1 mM Tris. The efficacy of Tris is shown to be due in part to the establishment of an optimum pH but an "amine effect" is also identified. The optimum pH for growth in vitro may be different from that encountered on the stigma. Atmospheric concentration of CO2 and pollen population density are also shown to have pronounced effects on germination in vitro.Germination in vitro is not significantly affected by prior incubation on the stigma surface. Neither crossnor self-stigmas exert any apparent influence on pollen viability in this respect. Similarly the numbers of tube initials produced in atmospheres of 98% relative humidity (RH) are not altered by such treatments. Conversely, incubation in germination medium and in 98% RH prior to pollination appears to modify pollen-stigma interactions to some extent.Stigma extracts added to the germination medium cause an increase in the rate of hydration of both cross- and self-pollen, but this effect is greater in the case of cross-pollen. Evidence for specific inhibition of self-pollen germination by stigma extracts added to the medium was not obtained. The relevance of these phenomena to current concepts of the mechanism of self incompatibility (SI) are discussed and a potential use of the medium in overcoming the SI response is suggested.