The effect of 63 quassinoids on eukaryotic protein synthesis has been investigated. Seventeen of the tested compounds showed potent in vitro activity, with IC50s below 2 microM for inhibition in Krebs ascites translation extracts. Sixteen of these quassinoids were also potent inhibitors of in vivo protein synthesis when exposed to Hela cells for 1 h. Our results led to the following structure-activity relationships for quassinoids regarding translation inhibition. Activity is influenced by (i) the nature of the C-15 side chain, (ii) the nature of A ring modifications, (iii) the presence or absence of a sugar moiety, and (iv) the presence of an epoxymethano bridge.