The major constituents from the fruits of Maclura pomifera are the prenylated isoflavones, osajin (1) and pomiferin (2). Their structures were elucidated using NMR spectroscopic techniques and mass spectrometric analysis. Compound 2 showed potential inhibitory activity in histone deacetylase (HDAC) enzyme assay. It also exhibited growth inhibitory activity on five human tumor cell lines and more sensitive inhibitory activity on the HCT-15 colon tumor cell line. Further structure-activity relationships of position 3 on ring B from aromatic ring will be reported in due course.