Matural curcuminoids, curcumin, I, II and III isolated from turmeric (Curcuma longa) were compared for their cytotoxic, tumour reducing and antioxidant activities. Curcumin III was found to be more active than the other two as a cytotoxic agent and in the inhibition of Ehrlich ascites tumour in mice (ILS 74.1%). These compounds were also checked for their antioxidant activity which possibly indicates their potential use as anti-promoters. The amount of curcuminoids (I, II and III) needed for 50% inhibition of lipid peroxidation was 20, 14 and 11 g/m. Concentrations needed for 50% inhibition of superoxides were 6.25, 4.25 and 1.9 micrograms/ml and those for hydroxyl radical were 2.3, 1.8 and 1.8 micrograms/ml, respectively. The ability of these compounds to suppress the superoxide production by macrophages activated with phorbol-12-myristate-13-acetate (PMA) indicated that all the three curcuminoids inhibited superoxide production and curcumin III produced maximum effect. These results indicate that curcumin III is the most active of the curcuminoids present in turmeric. Synthetic curcumin I and III had similar activity to natural curcumins.