The dose-response relationships of dicentrics and excess acentrics were analysed after exposure of human lymphocytes to a mixed fission neutron-gamma-ray beam. From the analysis of exclusively first division cells a linear-quadratic relation was obtained for dicentrics with the ratio of linear and quadratic components, zeta, equal to 2.76 Gy. Over the range of doses studied (0.04-1.97 Gy) intratrack events therefore predominated. This also applied to acentrics which were linearly related to dose. At the lowest level of observed effect and dose, r.b.e. values with respect to 60Co gamma-rays of up to about 11 were derived for dicentrics and acentrics. With increasing neutron dose the r.b.e. decreased.