The monoterpenes represent a potentially new class of breast cancer therapeutic agents. We have shown that d-limonene induces the regression of advanced rat mammary adenocarcinomas. These regressing tumors have an increased cellular concentration of both the mannose-6-phosphate/insulin-like growth factor II receptors and transforming growth factor beta 1. The terpene-induced regression of mammary tumors may result in part from the mitoinhibitory and differentiation properties of active transforming growth factor beta 1. Furthermore, the activation of transforming growth factor beta 1 in these tumors is likely to be facilitated by the increased concentration of the mannose-6-phosphate/insulin-like growth factor II receptors in the mammary tumor cells. Tumors not responding to terpene therapy lacked a rise in the mannose-6-phosphate/insulin-like growth factor II receptor level which may relate to the fact that this gene is hemizygous due to maternal imprinting.