In order to establish the validity of an algal assay procedure for the determination of algal nutrient levels in surface waters a suitable protocol was designed and followed by eight laboratories. This group consisted of one government laboratory, four university laboratories and three industrial laboratories. The basic procedure was to evaluate by use of the 'bottle' or batch test the precision and reproducibility of the growth response of one test organism, Selenastrum capricornutum, in four media of varying nutrient strength. The medium was originally defined for the PAAP test and modified slightly in subsequent evaluations. The test media of this experiment were all dilutions of the PAAP medium.