Currents were elicited by acetylcholine (ACh), by d-tubocurarine (dTC), and by mixtures of ACh and dTC, from stably transformed fibroblasts that express fetal or adult muscle nicotinic receptors. dTC acted as an antagonist of ACh for activation of adult-type receptors, whereas it acted as a weak partial agonist at fetal-type receptors. The channel- blocking action of dTC was not apparent at the concentrations used. The partial agonism could explain previous observations that dTC is less effective at blocking the responses of fetal-type receptors than adult- type receptors. Binding of dTC to receptors was independently assayed by measuring the reduction of the initial rate of binding of iodinated alpha-bungarotoxin. Binding of dTC to the two types of receptor was indistinguishable. The dose-effect relationship for the interaction of dTC and ACh at fetal receptors is consistent with the affinities of dTC measured in binding experiments.