Adult T-cell leukemia/lymphoma (ATL) is a malignancy of mature peripheral T lymphocytes caused by human T-cell lymphotropic virus type 1 (HTLV-1). There are an estimated 5 million to 20 million HTLV-1-infected individuals worldwide; their lifetime risk of developing ATL is 3-5 %, and high HTLV-1 proviral loads have been shown to be an independent risk factor. Although conventional chemotherapeutic regimens used against other malignant lymphomas have been administered to ATL patients, the prognosis is often poor. In previous studies, we screened 459 extracts from 344 plants to isolate components exhibiting antiproliferative activity against HTLV-1-infected T-cell lines (MT-1 and MT-2). In our continuing search for potential anti-HTLV-1 natural products, 15 extracts of Asclepiadaceae plants were further tested against MT-1 and MT-2 cells. The MeOH extract of aerial parts of Tylophora tanakae showed antiproliferative activity. Activity-guided fractionation resulted in the isolation of 6 phenanthroindolizidine alkaloids (including a new compound), and we examined their antiproliferative activity against MT-1 and MT-2 cells. The EC50 value of some of the alkaloids was in the low nanomolar range, comparable to that of the clinically used antineoplastic drug doxorubicin. Structure-activity relationship analyses suggested that a 14β-hydroxy moiety is essential for activity against HTLV-1-infected T cells. In contrast, the presence of a 2-methoxy moiety, a 7-methoxy moiety, or an N-oxide moiety appears to reduce the potency of the antiproliferative activity against HTLV-1-infected T cells. © 2015 The Japanese Society of Pharmacognosy and Springer Japan.