A study of the ultrastructure of cultured tobacco callus phloem shows that it is very similar to that found in the intact stem.Fibrils similar to the P protein, characteristic of angiosperm phloem, are found initially within the rough endoplasmic reticulum. This has not been shown previously.Preincubation of the callus at 0 or 50° C or in solutions of cholchicine prior to fixation have no effect on the relative proportions or the individual structures of the two forms of P protein found in tobacco callus phloem sieve elements. However 1% colchicine does reduce the number of microtubules found in callus cells by up to 80%.The morphological similarity between P 1 protein tubules and microtubules therefore appears coincidental and not due to a different arrangement of a common basic unit.The function of the P protein is considered and reasons for the comparative stability of plant microtubules compared with their animal equivalents are discussed.