In the unicellular red algae Porphyridium cruentum and P. aerugineum the phycobiliprotein content of the plastids is regulated by the applied energy fluence rate. Cells cultured at low energy fluence rates (220 μW cm(-2)) posses up to three times more phycobiliproteins than cells grown at high energy fluence rates (3200 μW cm(-2)). These values were obtained by direct measurement of the apoprotein of the phycobiliproteins. Transfer of cells from low to high energy fluence rates and vice versa results in an adaptation of the phycobiliprotein content to the new light conditions. This process starts immediately after the transfer of the cells and requires several days. On the other hand, the amount of the enzyme ribulose-1,5-bisphosphate carboxylase, which is also a prominent protein of the plastids of red algae, does not change significantly in response to differing fluence rates.