A range of water-soluble fluorescent dyes and dye conjugates have been injected into cells in Elodea canadensis Michx. leaves. All compounds are unable to cross the plasmalemma between living cells and the external solution, are not degraded to other fluorescent compounds by tissue homogenates, and do not affect cytoplasmic streaming. Despite being unable to cross the plasmalemma, molecules up to 874 dalton pass from cell to cell, smaller molecules showing greater mobility. The conjugate of fluorescein isothiocyanate and leucyl-diglutamylleucine (874 dalton) appears to be close to the limit for movement: in only three out of 17 injections was any movement visible; this movement was only to adjacent cells and was close to the limit of detection. Dye molecules of 1678 dalton and larger did not pass from cell to cell. From the relationship between the size of the dye molecules, measured using molecular models, and their intercellular mobility, the equivalent pore diameter of the Elodea leaf plasmodesmata has been estimated to lie within the range 3.0-5.0 nm.