Natural plankton samples were collected within June-November 1988 in the Southern Baltic. Species composition was determined and chlorophyll extracts from the samples were analysed for chlorophyll and copper content. The Cu content in extracts from those samples dominated by phytoplankton was considerable, especially during bloom (Cu/Mg ratio of up to 0.1), in contrast to samples with zooplankton predominance (zero Cu/Mg ratio). The data confirm results of previous studies on the blue-green alga Anabaena variabilis, proving the formation of a copper-chlorophyll complex in algal cells cultured in a high Cu concentration medium. Effects of such factors as light, pH, Cu and Fe concentrations in the medium on Cu incorporation were studied in batch cultures of the blue-green algae A. variabilis and Anacystis nidulans. The experiments showed replacement of Mg by Cu to occur in live algal cells, the process requiring light to occur. Moreover, certain Cu concentrations stimulate an increase in chlorophyll content of the blue-green algal cultures. Fixation of some molecules of chlorophyll by copper such that the cell has to produce new chlorophyll molecules in order to function normally cannot be excluded.