The effects of two microtubule-specific drugs, taxol and colcemid, upon the cell shape and cytoskeleton of several types of cultured fibroblastic cells were compared. While colcemid depolymerized completely the whole microtubular system, taxol induced decentralization of this system, leading to formation of numerous free microtubules filling the central cytoplasm. Morphometric determinations of two cell shape parameters, dispersion and elongation (G. Dunn and A. Brown, J. Cell Sci. (1986) 83, 313-340), have shown that, in all the tested cultures, taxol induced significantly larger decreases of average dispersion than colcemid; in addition, most taxol-treated cells, but not colcemid-treated ones, developed circumferential bundles of actin microfilaments instead of straight bundles. These results show that decentralization of the microtubular system, in contrast to its complete depolymerization, leads to the transformation of a polarized "fibroblast-like" cell morphology to an "epithelioid" morphology characterized by the smooth discoid cell shape and a circular actin pattern. Possible mechanisms of this transformation are briefly discussed.