The dynamic response of concrete floors to the passage of groups of up to 32 people was investigated. The floors were of flat slab construction and situated on the first-floor of a test building. An accelerometer and a displacement transducer were used to measure the vertical motion in the centre of the floor as different numbers of people either walked at a random pace or at a prescribed pace across the floor. The floor response increased with increasing group size, irrespective of the walking pace. For a given group size, walking at a pace to generate resonance did not always result in the largest response. In general, the maximum accelerations from a group were approximately double those generated by an individual. The implications of this observation on the serviceability acceptability criterion of BS 6472 are discussed. 15 refs.