High-field magnetoresistance and Shubnikov-de Haas (SdH) effects were studied in the A15 compound Nb3Sb in fields up to 215 kG. A change in the field dependence of the magnetoresistance for certain field directions above about 150 kG appears to signal the onset of magnetic breakdown. Five sets of SdH frequencies were observed, four of them closely corresponding to de Haas-van Alphen (dHvA) frequencies observed by Arko et al. The fifth frequency had an extremely large amplitude, about 20% of the backgroud magnetoresistance, and it is suggested that this also is due to magnetic breakdown. The results are compared with the ab initio band calculations of van Kessel et al., which can explain many of the observed features of the dHvA and SdH frequencies. © 1980 Plenum Publishing Corporation.