A method in which minute volumes of initially unknown size are used to monitor reactions has been developed. It is applicable whenever 32P-labeled substrates can be distinguished from 32P-labeled products by a filter assay. Three determinations are required: the Cerenkov radiation emitted (1) by the unknown aliquots affixed to filters and dried; (2) by the identical filters after processing to remove either substrates or products; and (3) by a precisely measured, necessarily much larger, aliquot in solution. Given the value obtained in (3) and the efficiency with which Cerenkov radiation can be detected in the dry state (25%) compared with the liquid state (56%), the unknown volumes, together with their acid-insoluble radioactivity, can be calculated from (1) and (2), respectively. Since the known volume counted in solution is not lost to further manipulations, the entire procedure uses less than 0.5 microliter.