Haemolytic substances produced by ichthyotoxic algae often are unknown in molecular structure or specific mechanism of toxicity. Detection and quantification of such substances are dependent on bioassays, using markers that are sensitive for haemolytic impairment and generation of a recordable response. The erythrocyte lysis assay (ELA) represents an advantageous bioassay in this respect, because the lytic response can be measured photometrically by the amount of released haemoglobin. The aim of the present study was to establish an improved assay based on the ELA principle, for sensitive determination of haemolytic substances of microalgae and for high sample throughput. For this purpose we adapted the ELA to a 96-well microtitre plate format, which significantly reduced the sample volumes and allowed rapid processing of samples. Further improvement was achieved by measuring absorption of lysed erythrocytes at 414 nm, which significantly increased the sensitivity of the ELA compared to the measurements at 540 nm that are usually applied in this type of assay. Using carp (Cyprinus carpio) erythrocytes it was possible to detect haemolysis induced by 4 μg ml-1 of saponin and as little as two haemolytic Alexandrium tamarense cells. It is suggested that this improved ELA in microtitre plates be used as a low-cost monitoring tool for detection and analysis of potential harmful algae. Furthermore, this ELA can be useful as a sensitive screening system for substances of pharmacological interest, e.g. selectively acting cytolytic antibiotics. Copyright © 2001 John Wiley & Sons, Ltd.