The effects of BAY u9773 (6(R)-(4′-carboxyphenylthio)-5(S)-hydroxy-7(E),9(E),11(Z),14(Z)-eicosatetraenoic acid), a cysteinyl- leukotriene analogue, were investigated on a variety of smooth muscle preparations in order to determine its profile as a cysteinyl-leukotriene receptor antagonist. The tissues were contracted with leukotriene C₄ or leukotriene D₄ and their receptor characteristics defined as either ‘typical’ or ‘atypical’ accordingto the activity or inactivity, respectively, of the selective antagonists ICI 198615, MK 571 and SKF 104353. BAY u9773 antagonised ‘typical’ cysteinyl-leukotriene receptors with pA₂ (or pK_B) values in the range 6.8–7.4 and also antagonised ‘atypical’ receptors with pA₂ values in the range 6.8–7.7. However, BAY u9773 had no effect at 10⁻⁶ M against a selection of non-leukotriene stimuli in the same preparations. BAY u9773 competitively displaced [³H]leukotriene D₄ binding to guinea-pig lung homogenate, with a pK_i of 7.0 ± 0.1. In the guinea-pig lung strip, BAY u9773 was found to be inactive at 10⁻⁶ M against leukotriene C₄- and leukotriene D₄-induced contractions, which may suggest the existence of a third type of cysteinyl-leukotriene receptor. These data demonstrate that BAY u9773 is a selective cysteinyl-leukotriene receptor antagonist with comparable activity at both ‘typical’ and ‘atypical’ receptors and as such represents a valuable tool for the study of cysteinyl-leukotriene receptors.