Abstract: A method and an apparatus for isolating and measuring the intensity of gamma radiation or other type of radioactive radiation from a radiation active deposit contained within a liquid sample, in which the measurement is performed in step with a previous isolation of a sample by an intermittent advance (10) of the sample from an isolation station to a measuring station. The measurement is made in step with the intermittent advance of the sample in a plurality of immediately succeeding partial measurements in a measuring unit which comprises a plurality of successively arranged radiation detectors (36) corresponding to a predetermined number of partial measurements. Each partial measurement constitutes a part of the total measurement of the same corresponding to the step of advance. The measuring results from the individual partial measurements of the sample are summed up (60), and the sum is registered. The sample may be deposited on disc-shaped membrane filter carriers.