A presumably new structure that we have called "globular smooth-tubule aggregate" was found in the cytoplasm of chondrocytes of 4 thanatophoric dwarfs. To the best of our knowledge these sharply delineated, rounded, sieve-like bodies of distinctive morphology have not been reported to occur in any other site or situation. Hence these structures may be of some diagnostic value. Continuity between the rough endoplasmic reticulum and smooth tubules in the aggregates was detected. Therefore it is suggested that smooth tubules in these aggregates derive by growth and extension from the rough endoplasmic reticulum.