The aim of this work was to investigate the sublethal neuropathic effects of tricresyl phosphate (TCP: mixed isomers), triorthocresyl phosphate (TO:CP) and triparacresyl phosphate (TP:CP) on differentiating mouse N2a neuroblastoma cells. This was achieved by a combination of measurements of cell viability, axon outgrowth and the levels of cytoskeletal proteins detectable on western blots of extracts from cells induced to differentiate in the presence and absence of the compounds. In a time-course experiment TCP inhibited the outgrowth of axon-like processes following exposure times of 24 h or longer. Dose-response experiments indicated that TCP and TO:CP exhibited similar sustained levels of toxicity following both 24 and 48 h exposure, with no significant difference between their respective IC(50) values. By contrast, TP:CP demonstrated a transient effect on the outgrowth of axon-like processes, which was detectable after 24 but not 48 h of exposure. Isomer-specific patterns of toxicity were also evident at earlier time-points, with only the ortho isomer showing significant levels of inhibition of axon outgrowth following 4-8 h exposure. Probing of western blots with antibodies against cytoskeletal proteins indicated that the inhibition of axon outgrowth by these compounds was associated with a sustained reduction in the levels of phosphorylated neurofilament heavy chain. The inhibitory effect on axon outgrowth of TO:CP but not TP:CP was enhanced in the presence of a microsomal activation system. Since TO:CP is the most neuropathic of the isomers of TCP in vivo, differentiating N2a cells provide a useful cellular system for mechanistic studies of the neurodegenerative effects of this organophosphate.