Radiophotosynthesis has been carried out using the unicellular green algea Scenedesmus acutus grown, as a substrate for preparing some carbon-14 labelled amino acids. Gaseous14CO2, in an air tight photosynthesis chamber or NaH14CO3solution, in an ordinary photosynthesis chamber, were used as radioactive carbon sources. The yields, radiochemical yields and specific activities of the formed radioactive products are reported in both cases. The results obtained clearly showed the advantages of using the NaH14CO3method. In that case the process was by far less tedious. Moreover, the chemical and radiochemical yields of the formed amino acids were relatively much more higher than the values obtained on using14CO2in the tight photosynthesis chamber.