Callus was initiated from seedling hypocotyls and mature stems of Papaver somniferum and maintained in culture. The ultrastructure of callus cells resembled parenchyma of intact plants, their most conspicuous feature being a large central vacuole. Meristemoids were formed at random on the surface of calli transferred to the appropriate medium. In cross section, meristemoids appeared as an inner meristematic region surrounded by several layers of concentrically arranged cells. Cells of meristemoids were smaller, more densely cytoplasmic, and more compactly arranged compared with callus. Roots and shoots induced to form from callus possessed cells that were morphologically and developmentally similar to laticifers in intact plants. Laticifers in redifferentiated organs, like those in intact plants, contained numerous capped vesicles that are apparently derived from stacked endoplasmic reticulum. Cell wall perforations between adjacent laticifer elements in redifferentiated organs developed by the gradual, simultaneous thinning of walls at the perforation sites.