Two naturally occurring isothiocyanates, Allyl isothiocyanate (AITC) and phenyl isothiocyanate (PITC) were investigated for their antioxidant and anti-tumour peroperties. Both AITC and PITC showed antioxidant and tumour reducing activities when administered intraperitoneally at a dosage of 25 microg/dose/animal for 5 consecutive days. The inhibition of PMA induced superoxide generation by the peritoneal macrophages in AITC and PITC treated animals were 34% and 30.3% respectively. Nitrite production by the peritoneal macrophages was also inhibited by the administration of both AITC (51.6%) and PITC (34.53%). Administration of these naturally occurring isothiocyanates also inhibited the lipid peroxidation (AITC - 47.4%; PITC-25.9%) in mice liver homogenate. Both isothiocyanates also scavenged hydroxyl radicals (OH-) in vitro. Percentage inhibition of OH-production in AITC treated animals were 61.9% and that of PITC treated animals were 69.7%. Treatment with these isothiocyanates reduced the solid tumour development induced by Dalton's lymphoma ascites (DLA) tumour cells. The life span of Ehrlich ascites tumour bearing animals was also significantly enhanced to 152.4% and 95.2% by the treatment with PITC and AITC, respectively, implicating the anti-tumour activity of these compounds.