Fertilized Lytechinus eggs exposed to the volatile anaesthetic halothane before metaphase do not undergo cytoplasmic cleavage. This effect has been correlated with the failure of the contractile ring to assemble. Comparative studies on mitotic apparatuses isolated from control and halothane-treated cells show that halothane significantly impairs both spindle and aster growth as early as metaphase. When transferred to control solutions, halothane-treated cells initiate furrowing activity in association with either the first or second mitotic division, depending on the duration of the exposure to anaesthetic and the concentration employed. In contrast to these effects, halothane has no effect on any aspect of the cleavage process if applied later than metaphase. In this case, furrows develop even in the presence of halothane, deepen progressively, and complete cell division. These observations confirm and extend previous studies on echinoderm eggs exposed to volatile anaesthetics, and support the view that anaesthetic agents indirectly prevent cell cleavage by inhibiting the growth of mitotic apparatus.