Oat aleurone protoplasts, maintained in sterile liquid culture for 5 d, are able to take up a number of fluorescent probes of varying charge and of molecular weights in the range 457 to 637. In addition to Lucifer Yellow CH, these include PTS, HPTS, Lucifer Yellow AB, calcein, and sulphorhodamine-101, most of which have previously been described as membrane-impermeant due to their physicochemical properties. The transport of these probes across the plasma membrane and their subsequent sequestration within the vacuole, is inhibited by the drug probenecid, negating the possibility that movement is solely by simple diffusion. In contrast, Trypan blue (mol. wt. 961) is excluded by all live oat aleurone protoplasts. The uptake of carboxyfluorescein into protoplasts during the early stages of development can, in part, be explained by diffusion of the undissociated molecule and subsequent anion trapping in the cytosol. However, both the uptake into the protein bodies of 1-d-old protoplasts and into the vacuoles of 5-d-old protoplasts is inhibited by probenecid. This indicates that the transport of carboxyfluorescein is carrier-mediated and that the carrier is present on the tono-plast membrane throughout protoplast development. Since probes such as carboxyfluorescein have physicochemical properties similar to some phloem-mobile xenobiotics, the results have important implications for theories pertaining to the movement and compartmentation of xenobiotics within plants.