Harmful algal blooms represent a major threat to marine production, and particularly to shellfish farming. Current methods for analyzing environmental samples are tedious and time consuming because they require taxonomists and animal experiments. New rapid detection methods, such as immunoassays, are sought for alerting purposes and for the study of algal ecodynamics in their natural environment. Alexandrium minutum, which causes paralytic shellfish poisoning, occurs with increasing frequency along European coasts. We have developed a one step immunochromatographic assay which is based on the principle of immunochromatographic analysis and involves the use of two distinct monoclonal antibodies directed against surface antigens of A. minutum. The primary specific antibody was conjugated with colloidal gold, and the secondary antibody (capture reagent) is immobilized on a strip of nitrocellulose membrane. We could demonstrate that whole algae are able to diffuse without restriction in the porous material. The assay time for this qualitative but highly specific assay was less than 15 min, suitable for rapid on-site testing. (C) 2009 Elsevier B.V. All rights reserved.