The organisation of microtubules rich in post-translationally modified alpha-tubulin has been investigated in a fibroblast cell line (NIH-3T3-T15) that can be reversibly transformed. An immunofluorescence microscopy study of the static non-transformed cells has revealed a central distribution of wavy microtubules showing post-translational modifications. When transformed there is a marked increase in cell motility and the appearance of long thin cytoplasmic 'tails'. These tails have been found to contain conspicuous bundles of post-translationally modified microtubules that run down the length of the processes and terminate close to the plasmalemma. Both detyrosinated and acetylated alpha-tubulin are present as major species in these modified microtubules. Such a pattern of modified microtubules is only occasionally seen in the untransformed NIH-3T3-T15 cells. We have also found them to be present in other transformed fibroblast lines. The presence of bundles of microtubules rich in modified alpha-tubulin in the cell tails is correlated with a marked reduction in the numbers of F-actin stress fibres. The possible role of these modified stable microtubules in cell motility is discussed.