The crude extract of Calythropsis aurea (Myrtaceae) produced a pattern of differential cytotoxicity in the NCI 60 cell line assay which was similar to those of known tubulin-interactive compounds. Cytotoxicity-guided fractionation led to the isolation of two new chalcones, calythropsin [1] and dihydrocalythropsin [2], which were responsible for the activity. Calythropsin was demonstrated to have a weak effect on mitosis, and presumably also on tubulin polymerization.