A comparison has been made of the effectiveness of eicosapentaenoic (EPA) acid administered as either the free acid or the ethyl ester as an anticachectic and antitumour agent in mice bearing an experimental cachexia-inducing tumour (MAC16 colon adenocarcinoma). While the free acid of EPA was effective in reversing host body weight loss and inhibiting tumour growth the ethyl ester was ineffective in either respect at the same dose level, even when administered with a high fat diet. The lack of effectiveness of the ethyl ester correlated with the inability to reach effective plasma and tumour concentrations of EPA over the initial time period. Whereas effective plasma concentrations of EPA were achieved within 24 h after administration of the free acid, a time lapse of 96 h was required with the ethyl ester, even when combined with a high fat diet. Due to the acuteness of the MAC16 model this time is too long for a therapeutic benefit to be realized.