Directionally selective retinal ganglion cells respond strongly when a stimulus moves in their preferred direction, but respond little or not at all when it moves in the opposite direction. This selectivity represents a classic paradigm of computation by neural microcircuits, but its cellular mechanism remains obscure. The directionally selective ganglion cells receive many synapses from a type of amacrine cell termed 'starburst' because of its regularly spaced, evenly radiating dendrites. Starburst amacrine cells have a synaptic asymmetry that has been proposed as the source of the directional response in the ganglion cells. Here we report experiments that make this unlikely, and offer an alternative concept of the function of starburst cells. We labelled starburst cells in living retinas, then killed them by targeted laser ablation while recording from individual directionally selective ganglion cells. Ablating starburst cells revealed no asymmetric contribution to the ganglion cell response. Instead of being direction discriminators, the starburst cells appear to potentiate generically the responses of ganglion cells to moving stimuli. The origin of direction selectivity probably lies with another type of amacrine cell.