Epithelio-mesenchymal transition, which involves the re-organisation of cell-cell adhesion molecules and the actin cytoskeleton, can be induced in embryonic neural epithelium in vitro by protein kinase-C inhibitors. A non-inhibitory analogue, BIM V, and potent inhibitors of other kinases are not active. This suggests a central role for C-kinases, although the powerful specific C-kinase inhibitors BIM I and Ro 31-8220 show lower than expected activity. Co-inhibition by several kinases is unlikely to account for this, since no potentiation occurs when these are combined with potent inhibitors of other kinases. BIM I and Ro 31-8220 strongly inhibit only conventional calcium-regulated C-kinases; this and the lack of effect of TMB-8, which inhibits calcium release, suggests that novel and/or atypical isoforms are involved. Various potentiators and activators of conventional and novel C-kinases have no obvious effect alone and fail to reduce the effect of staurosporine, suggesting that atypical C-kinases are critical. The presence of C-kinase isoforms in the E2 embryonic neural tissues has been probed on Western blots, revealing immunoreactivity for the atypical isoforms iota (or lambda) and zeta and the alpha, gamma, epsilon and mu isoforms. Immunofluorecent localisation on sections of embryos has shown the widespread distribution of conventional and novel isoforms but only the atypical isoforms lambda and zeta are enriched at the apical margins of the neural and other epithelia; they overlap with the cell-cell adhesion molecule N-cadherin and with F-actin. Thus, epithelio-mesenchymal transition in the embryonic neural epithelium in vitro is induced by inhibiting protein kinase activity, probably via an atypical protein kinase-C; atypical protein kinase-C isoforms are present in the tissue at the appropriate developmental stage and subcellular site in cells capable of epithelio-mesenchymal transition.