A series of experiments has been undertaken in order to test the biological properties of neutrons produced in the cyclotron of the Institute "Ruder Bosković" (IRB) in Zagreb. Protective effect of AET (2-amino ethylisothiuronium bromide hydrobromide) on survival of L cells irradiated by fast neutrons generated in the IRB cyclotron were studied by employing the single cell clonal growth method. For comparison the protective effect of AET after gamma irradiation has also been studied. The most important findings that have emerged from these experiments can be summarized as follows: (1) Protective effect of AET was present after gamma irradiation only. (2) The degree of protection was dependent on AET concentration in the growth medium. (3) No protective effect was found after neutron irradiation. These findings are in agreement with the generally less efficient protection of this compounds after high-LET irradiation.