In an effort to compare the primary production (PP) measurements in the Baltic Sea, four institutes got together in an intercalibration exercise with the aim to obtain similar values and a common method protocol. The strategy was to compare different methods on the same water sample and to identify sources to any differences. The four methods showed different results and the differences were systematic. This was due to that the methods measured different things and to that there were differences in the manuals followed as well as to differences in the measurements. The manuals gave also possibilities to choose different variations of the method. We have now managed to list all the differences and have a plan to investigate each step further with the aim to agree on a common method. This needs however to be tested to find the best method. The first step is to carefully measure the spectral composition of our incubators. We will see to that we get proper spectra and enough light. Other differences to be tested are the incubation time, the quality of 14C added and the end addition of hydrochloric acid, HCl. We think that the measurement of primary production is important and want to do it in the best possible way. To do this we need to have intercalibrations or a regular basis. We also need to test all the steps to find the most suitable method. The discussions will continue until a common manual is agreed upon. We also need to invite institutes from around the Baltic Sea to agree on a change in the common manuals.