Brosimum alicastrum Swingle (Moraceae)is a large, evergreen canopy tree found throughout the Neotropics and is native to Guatemala and Costa Rica. The fruit of the tree is also known as "Mayan bread fruit" or breadnut. The seeds and the fruit of the tree have been used in traditional Mayan medicine for the treatment of menopause and osteoporosis, and are thought to have hormone-like activities. Brosimum alicastrum Swingle (Moraceae)was identified and collected under an international agreement with the University of San Carlos, and the University of Costa Rica. The fruit and leaves of the tree were collected, dried, extracted and tested in several in vitro assays to determine their biological activities, particularly for their potential estrogenic and antiestrogenic effects. Methanol extracts of the fruit displaced 3H-estradiol binding from the estrogen receptors (ERa and ERß, EC50 48 and 49 µg/ml). Methanol extracts of the fruit pulp including seeds, and an extract of the leaves (20 µg/ml)induced a SEAP reporter gene in transiently transfected MCF-7 cells, which was inhibited by the ER antagonist ICI 182,780 indicating estrogenic effects were mediated through the ER. Conversely, a methanol extract of the fruit pulp alone was not estrogenic as compared with DMSO controls, and reduced SEAP reporter gene activity in a concentration dependent manner, suggesting potential anti-estrogenic effects.