Sea urchin primary mesenchyme cells: ingression occurs independent of microtubules. The formation of primary mesenchyme cells in euechinoid sea urchin embryos involves the transformation of 32 epithelial cells into mesenchymal cells in a process referred to as ingression. The mechanism that drives this epithelial-mesenchymal transformation has yet to be identified. Previous studies (J. R. Gibbins, L. G. Tilney, and K. R. Porter, 1969, J. Cell Biol. 41, 201-226; L. G. Tilney and J. R. Gibbins, 1969, J. Cell Biol. 41, 227-250) implicated that microtubules are essential components for the normal development, including ingression, of the mesenchymal cells. In the present study I have reinvestigated the role of microtubules in ingression by using the microtubule-disrupting drugs colchicine and nocodazole, and the microtubule-stabilizing drug taxol. The effect of these drugs on microtubules was monitored by indirect immunofluorescence using monoclonal antibodies specific for alpha- and beta-tubulins. The microtubule array seen in control embryos disappeared in colchicine- and nocodazole-treated embryos, while it was enhanced in taxol-treated embryos. When premesenchyme blastulae of Strongylocentrotus purpuratus were treated with any of these reagents the primary mesenchyme cells ingressed on schedule and appeared to undergo cell-shape changes identical to those observed in untreated embryos. The conclusion of this study is that the mechanism of primary mesenchyme cell ingression does not include an essential role for microtubules; ingression occurs regardless of the presence or absence of microtubules.