Sister chromatid exchange (SCE) frequencies in « in vitro » cultures of human lymphocytes were analysed after exposure to non toxic concentrations of epichlorohydrin (1×10−5M, 1×10−8, 1×10−11M). High sensitivity of SCE test allowed to point out a gradual increase of SCE frequency. In particular, 1× 10−8MECHH, which had proved unable to increase the frequency of structural chromosome aberrations, was indeed able to induce a significant increase of SCE.