The aims of this project were to investigate the cellular stress response (CSR) in Enteromorpha spp. and assess the potential of the Stress-70 protein (an indicator of the CSR) as a biomarker of pollutant exposure and acquired tolerance in Enteromorpha spp., compared with conventional physiological endpoints of toxicity. Cross-reactivity of a commercial Stress-70 antibody with E. intestinalis proteins was determined and used to develop an assay for Stress-70. Using this assay E. intestinalis was found to exhibit a typical heat shock response. Stress-70 proved to be a relatively insensitive biomarker of copper exposure and did not appear to be involved in copper tolerance, the genetic basis of which was investigated by growing E. intestinalis using a novel culturing technique. Although growth was variable, it provided a simple, consistent and sensitive measure of copper toxicity. The chlorophyll fluorescence parameter Fv/Fm was insensitive to copper exposure. Nutrient limitation enhanced copper toxicity and significantly impaired growth, Fv/Fm and Stress-70 production in E. intestinalis. In both copper ‘sensitive’ and ‘tolerant’ E. intestinalis, copper exposure did not affect the ability to raise a heat shock response. In ‘sensitive’ algae, copper and heat shock were additive stressors, with heat shock acting as a stronger inducer of Stress-70. Only heat shock affected 'tolerant' algae. Zinc was less toxic than copper but in contrast to copper studies, Stress-70 was a relatively sensitive indicator of zinc exposure, compared to Fv/Fm and growth. Studies of triazine herbicides revealed that on a molarity basis, Irgarol 1051 was more toxic to E. intestinalis than atrazine. Fv/Fm and growth were strongly affected by Irgarol exposure, but Stress-70 levels were unaltered by exposure to the herbicide. Fv/Fm and Stress-70 were poor in situ biomarkers of pollution, but another chlorophyll fluorescence parameter - complementary area - appeared to correlate with levels of organic pollution. Overall, Stress-70 was found not to be a useful biomarker of exposure to copper or triazines in E. intestinalis, or in situ pollution. However, the Stress-70 assay developed has a number of alternative applications and Enteromorpha spp. were deemed to be potentially useful in pollution monitoring with the selection of suitable biomarker responses.