The effects of Pseudokirchneriella subcapitata exudates on the acute toxicity of metals (Cd, Cu, Pb and Zn) were studied using (1h) algal photosynthesis inhibition tests. The metal concentrations tested were 30, 60, 120, 250 and 500microgL(-1) for Cd and Zn; and 500, 1000, 2000, 3000, 4000microgL(-1) for Cu and Pb. It was established that P. subcapitata exudates markedly decreased metal toxicity. This effect was ascribed to: (i) the presence of organic ligands that bind metals and reduce the concentration of free ionic metal, and/or (ii) interaction of exudates with the cell surface with a subsequent decrease in metal uptake. The effects of the exudates on colloid aggregation were also examined using two different types of single particle counters (SPC). Exudates facilitated the colloid removal, likely via acceleration of aggregation (bridging). The results clearly demonstrate that algal exudates play an important role in the biogeochemical cycling of metals in natural surface water: (1) by reducing free metal concentrations and toxicity to living organisms and (2) by favoring colloid aggregation leading to the removal of colloid-bound metals (colloidal pumping). Such results highlight one potential application of the algae in the remediation of metal-contaminated waters. The results also suggest that current algal toxicity testing protocols, in particular long-term and static tests, may underestimate metal toxicity because of the presence of algal exudates.