The present invention relates to a protienaceous compound or functionally active derivative or part thereof having a binding site for a group represented by formula (I) which is part of a group of toxins derived from various cyanobacteria, to a method for its production, to diagnostic kits and to an affinty matrix (e.g. for use in immunoaffinity columns, online detection and purifications devices) containing the proteinaceous compound as well as to methods for substantially decreasing the amount of a compound containing the group represented by formula (I) in fluids or for concentrating compounds, e.g. toxins, containing the group represented by formula (I) from fluids such as crude water samples, extracts of algae or other tissue samples, e.g. to determine toxin concentrations.