Coated vesicles varying considerably in morphology have been described in various types of cells and a survey of the literature leads one to consider it probable that more than one functional entity has been given this name. Friend & Farquhar (1967) in a widely quoted paper present evidence for two types of coated vesicles in the rat vas deferens: a large type (> 100 nm in diameter) which they equate with heterophagosomes and to which they ascribe the functions of protein uptake and membrane resorption, and a smaller variety (< 75 nm in diameter) which they consider to be primary lysosomes. Kallio, Garant & Minkin (1971) described the ruffled border of active osteoclasts in fish and rats. They drew attention to deep imaginations and villous extensions of the plasma membrane which possess a coat of repeating units along their cytoplasmic surface. The presence of numerous smooth-walled and coated vesicles in the cytoplasm deep to the ruffled border was also noted. The authors suggest that the participate coat on the surface cytoplasmic membrane may be of a different nature to that present on the coated vesicles. Tangential sections of the coating on the cytoplasmic aspect of the surface membrane failed to reveal the polygonal pattern characteristic of the coated vesicles in the apical cytoplasm and it was thought possible that while the coated vesicles played a part in protein resorption, the particles on the ruffled border may represent sites of enzymatic activity connected with bone demineralisation.