The oestrogenic and antioestrogenic activities of a series of substituted derivatives of 1,1,2 triphenylbut-1-ene have been determined using primary cultures of rat pituitary gland cells to monitor prolactin synthesis in vitro. The relative binding affinity of the agonists for the oestrogen receptor was consistent with their oestrogenic potency. Bis para substitution at C1 of 1,1,2 triphenylbut-1-ene with either phenolic or acetoxy groups produced partial agonists. The antioestrogenic properties were reversible by the incubation of cells with increasing concentrations of oestradiol. The results lend support to a hypothetical single binding site model of oestrogen action, based upon an adaptation of Belleau's macromolecular perturbation theory.