The fluorochromatic reaction that viable pollen grains exhibit when exposed to fluorescein diacetate (FDA) is an easy, quick and accurate tool for assessing pollen viability in many plants. Despite its widespread use, the method as initially proposed by Heslop-Harrison and Heslop-Harrison lacks specificity in some respects that are essential for comparing simultaneously the viability of different pollen sources. We have determined the time needed for the fluorochromatic reaction to take place and fade, and the lifetime and most effective concentration of the FDA working solution. Our results show that consistent records of pollen viability in samples of olive (bicellular pollen grains) and cherimoya (tricellular pollen grains) can be obtained within minutes, but that measurement must be completed in the first hour after pollen exposure to FDA. The lifespan of the FDA working solution is 1 h, whereas concentrations between 1.5x10(-5) M and 1.8x10(-3) M are equally valid. Pollen grains of both species responded similarly, although some differences were observed in the persistence of fluorochromasia.