Six diterpenes, including two new natural products, were isolated from the seeds of Cephalotaxus harringtonia. The new metabolites were characterised as 8β-hydroxy-9(11),13-abietadien-12-one and 5,6-didehydroferruginol, while the known compounds were identified as ferruginol, sugiol, 6,12-dihydroxyabieta-5,8,11,13-tetraen-7-one, and abieta-8,11,13-trien-7β-ol. These compounds were studied in vitro for their antimicrobial activity against clinically isolated bacteria and Candida strains. Ferruginol and 6,12-dihydroxyabieta-5,8,11,13-tetraen-7-one showed antimicrobial activity against Gram-positive bacteria. None of the six diterpenes was active against the Gram-negative organisms and yeasts tested.