Two new monotetrahydrofuran Annonaceous acetogenins, montacin (1), and cis-montacin (2), along with four known acetogenins, annonacin, cis-annonacin, annomontacin, and cis-annomontacin, were isolated from the seeds of Annona montana. The structures of the new isolates were elucidated by spectral and chemical methods. The new acetogenins exhibited moderate in vitro cytotoxic activity against the 1A9 human ovarian cancer cell line. Interestingly, when Ca(2+) was present, compounds 1 and 2 became 3- to 10-fold more active against 1A9 cells and the PTX10 subline.