Anthers of Nicotiana tabacum (n = 24) were cultured on nutrient agar and examined at intervals for pollen embryoids. Embryoids were formed in anthers of varying developmental stage, the youngest of which coincided with the liberation of free microspores from tetrads, and the oldest with the formation of bicellular grains. This period in the development of the anther occupied 4–5 days. Older anthers within this range were more successful than younger anthers. The first mitosis of the pollen was typically asymmetric and resulted in the formation of unequal generative and vegetative cells. Some of the grains then went into a lag phase for at least 5–6 days, after which the mitotic condition was restored. Embryoids were formed by repeated division of the vegetative cell. If the generative cell divided, it did so only once or twice. Occasionally the first mitosis was symmetric and gave rise to equal cells, and in these instances both cells probably participated in embryoid formation. The youngest anthers examined were probably less successful because fewer grains survived to enter mitosis. The number of embryoids produced varied considerably from one anther to another both within the same bud and between different buds: values ranging from less than 400 to 10 000 per anther were encountered. Most of these degenerated after the first few divisions, partly because they burst prematurely from the pollen grain wall. Embryoids which continued to develop formed plantlets and/or callus. The largest number of plantlets obtained from one anther was 32. Haploid plantlets were also regenerated from callus by transferring it to a low-sugar medium without auxin. The behaviour of grains not forming embryoids was also noted.