The aim was to study the protective effects of fullerenols, C60(OH)x, on Stylonychia mytilus cells exposed to 60Co gamma-rays and the probable mechanisms of fullerenols protection. Ciliated protozoans Smytilus, kept in solutions of fullerenols at different concentrations, were irradiated with 60Co gamma-rays to various dose levels. Surviving cells were counted each day over 5 days after irradiation, and the surviving fraction was calculated. The relations of the surviving fraction to radiation dose and to fullerenols concentration were studied. Superoxide dismutase (SOD), catalase (CAT), malondialdehyde (MDA) and lipofusion (LIP) levels in S. mytilus were also measured. The surviving fraction of S. mytilus decreased with increasing gamma-ray doses from 100 to 2000 Gy. Fullerenols enhanced the surviving fraction, except for the highest gamma-ray dose level. The maximum protection by fullernols occurred at a concentration of 0.10 mg ml(-1). However, fullerenols at concentration of 0.25 mg ml(-1) yielded a surviving fraction lower than that for the control sample. Fullerenols at a concentration of 0.10 mg ml(-1) increased the SOD and CAT activities in the gamma-ray plus fullerenols (gamma + F) group compared with the levels in both the gamma-ray (gamma) group (p < 0.01) and the control group (p < 0.01). The MDA and LIP levels in the gamma + F groups (p < 0.01) were significantly lower than that in both the control group (p < 0.05) and the gamma group (p < 0.01). At a concentration of 0.25 mg ml(-1), fullerenols reduced the SOD and CAT activities, but increased the MDA and LIP level compared with the control. There was no significant difference in SOD and CAT activities between the gamma + F group and gamma group. While the MDA and LIP level in the gamma + F and gamma groups were similar at a dose of 500 Gy, the LIP level in the gamma + F group was significantly higher than that in the gamma group (p < 0.01) at a dose of 2000 Gy. Fullerenols are good radiation protectors for the protozoan S. mytilus exposed to gamma-rays. The effectiveness of radioprotection depends on both fullerenols concentration and gamma-ray dose. The protective effect of fullerenols on damage induced by gamma-rays seems to be mediated, at least in part, through their anti-oxidative and radical scavenging activities.