The shielding properties of the four‐foot concrete wall of the M.I.T. cyclotron have been studied, using foils for neutrons and x‐ray films for gammas. Fast neutrons were measured by counting the beta‐active products of Al27(n, p)Mg27, Fe56(n, p)Mn56, and Al27(n, &agr;)Na24reactions, thermal activation being eliminated by Cd holders. Resonance (1.44 ev) and thermal neutron distributions, measured by Cd‐In‐Cd and In, with suitable corrections, were reduced to absolute intensities by comparison with a known flux. Measurements were made with cylindrical concrete blocks fitted into a 4½″ diameter hole which extended through the concrete wall. The results indicate an attenuation factor of 8.3 per foot for the fast neutron component and 10 per foot for thermal neutrons. An estimate of the fast neutron flux above 4.5 Mev showed a transmitted intensity slightly higher than the presently accepted tolerance value when bombarding Be. The absolute thermal neutron flux was below the tolerance level. The r.m.s. distance and the diffusion length for thermal and resonance neutrons in concrete were estimated by considering the distributed source to be equivalent to a point source at the inner face of the wall. The gamma‐ray measurements were not convertible to roentgens. A mean energy of 2.7 Mev was obtained for the gamma‐radiation within the wall. The attenuation factor for total gamma‐radiation in the wall at large distances from the inner face was 5.9 per foot.