A dimorphism is observed in barley (Hordeum vulgare L., cv. Akka) pollen when stained with acetocarmine from the mid-binucleate stage onwards. The majority of grains have staining cytoplasms, while the remainder have cytoplasms which take up little or no stain (NS grains). The staining dimorphism cannot be detected at the late-uninucleate microspore stage when anthers are normally cultured, but the evidence suggests that the microspores have already diverged at this time and it is the cells destined to become NS grains in vivo that respond in culture to become pollen calluses. Evidence comes from a comparison of the frequencies of NS grains and pollen calluses and from their distribution between and within anthers.