Olfactory receptor responses (electroantennograms) were recorded from antennae of danaid butterflies. Antennae of male and female queen butterflies (Danaus gilippus berenice) respond equally strongly to the hairpencil of queen males, to its crude extract, and to one of its two identified secretory components (the ketone). Responses to the second component (the diol) are weak. Hairpencils of a related species, Lycorea ceres, which also contain the ketone, are equally effective in eliciting electroantennograms from both sexes of the queen. Antennae of another related species, the monarch (Danaus plexippus), respond to the same stimuli as does the queen. Monarch hairpencils, which lack the ketone, do not elicit electroantennograms in monarch or queen antennae.