The progress curves of 14C retention for samples of phytoplankton from the Irish Sea incubated at contrasting light intensities have been obtained by two methods. The first method (A) involved the incubation of the samples for various periods up to 6 h, while the second method (B) consisted of making a series of short-term incubations over the same 6 h period. Over this period, the cumulative uptake was generally less when estimated by Method A than by Method B. The difference was greater in the samples incubated at the lower light intensity, the light history of the samples having no effect on the difference. The differences had a kinetic basis, with two combinations of progress curves obtained by use of the two methods. The first combination was associated with samples collected in the early morning, while the second combination was exhibited by samples taken in the afternoon, irrespective of sampling depth. In certain samples, no increase in the 14C retained by the cells as measured by Method A was observed after 4 h. The cumulative retention of 14C by the cells after 2 h was generally greater when estimated by Method A than by Method B, this situation being reversed after 4 h. This reversal indicated a change in uptake kinetics between 2 and 4 h and it is suggested that this interval represents the time necessary for the 14C to work through the cellular pool of carbon. The findings are discussed in relation to the methodology for obtaining both estimates of primary production and 14C uptakelight intensity curves for marine phytoplankton.