A series of experiments on the different methods of measuring the relative uptake of C14, by the various size fractions of the phytoplankton, were carried out in a mangrove swamp of the estuarine type at 25º south latitude. As a complement, some factors which might influence the relative uptake of the different fractions in inshore waters, were studied. These factors are related, mainly, with the increase on the amount of labelled particulate matter during incubation period, the effect of different pressures in the filtration of the size fractions of the phytoplankton, the use of a range of volumes for the incubation of samples, and the effect of formalin on the fixation of samples, after the incubation period. Based on these results, some suggestions are made for the use of the C14 method in inshore waters.