Comparative studies have been made of the relative toxicities, in solution, of the metals cadmium, copper, cobalt, lead, mercury, silver, selenium, barium, and nickel to algae (phytoplankton). Several of these' metals have been considered potential pollutants in water bodies and one, copper, is used extensively as an algicide. Toxicity tests were performed under controlled conditions using growth as an indication of algal-metal response. Chlorella, Scenedesmus, Haematoaoacus and Chlamydomonas have been used as test species. Both the metals and the species differed markedly in their response. Silver was found to be the most toxic metal overall, inhibiting growth of Chlorella at levels of 0.005 ppm. Mercury and cadmium were also highly toxic to Chlorella i.e. 0.1 ppm Cu inhibited growth. Selenium, nickel and cobalt were somewhat less toxic, although inhibition and death of one or more species occurred at 0.1-5.0 ppm, whilst lead was apparently non-toxic at the levels used. Haematococcus was generally a good deal more resistant than other species. The response to increasing metal concentration was either a gradual diminution of growth or a drastic fall above a threshold level. This response was metal specific. Copper and nickel were found to act synergistically each enhancing the other's toxicity, but selenium was found to reduce the toxicity of cadmium thus showing metal antagonism. Potential ecological consequences are discussed.