AbstractUsing turbidometry, electron microscopy and immunofluorescent microscopy experiments we studied the effect of captan, a widely used pesticide on mammalian microtubules and microfilaments. Turbidometry at 350 nm showed a dose‐dependent inhibition of tubulin assembly incubated with captan. The pesticide, given at equimolar concentration with tubulin (30 μM), caused the total inhibition of microtubule formation, while at lower concentrations (5–20 μM) the inhibition of tubulin polymerization was less extensive. At the same concentration range (5–30 μM), captan also promoted the disassembly of performed microtubules. The results of thein vitroeffects of captan with microtubules were confirmed in parallel by electron microscopic studies.In vivo, captan caused also depolymerization of microtubules in cultured mouse fibroblasts as shown by indirect immunofluorescent staining of tubulin. The extent of microtubules disassembly was concentration‐ and time‐dependent. While incubation of the cells with 10 μMcaptan for 3 h disturbs totally the microtubular structures, incubation with 5 μMcaptan needs 12 h for the same effect. Recovery of microtubules was observed, when preincubated cells were extensively washed. No interaction of this drug with equimolar concentration of G‐ or F‐actin could be observedin vitro, as shown by polymerization experiments. In line with this, the fluorescent actin pattern in mouse fibroblasts incubated with 10 mMcaptan for up to 12 h did not seem to be altered. From these results it is concluded that captan interacts in equimolar concentrations with tubulin affecting the assembly and disassembly of microtubulesin vitroand in cultures