Single cells were isolated from leaves of pigeonpea ( Cajanus cajan) by a combination of enzymic digestion and mechanical agitation. When incubated with culture filtrates from isolate P3 of Phytophthora drechsleri f.sp. cajani, rapid cell death occurred, as assessed by the vital stain phenosafranine. This formed the basis of a cell bioassay to quantify the toxicity of crude and partially purified components of culture filtrates. The toxic activity was stabilized by dithiothreitol and 2-mercaptoethanol, but activity was partially lost on treatment with protease or storage at pH values which deviated from neutrality. Boiling completely destroyed toxic activity. A fraction with toxic activity (PPF) was separated from crude material by gel filtration. This fraction eluted at a volume corresponding to an apparent molecular weight of approximately 47 kD. Polyacrylamide gel electrophoresis of PPF revealed three major bands which stained positively for both protein (Coomassie blue) and carbohydrate (periodic acid-Schiff's reagent) with apparent molecular weights of 31, 36 and 48 kD. Protoplasts and cells were equally sensitive to PPF, and plant cuttings immersed in desalted culture filtrates showed symptoms similar to those of the disease. Growth of cell suspensions of pigeonpea was inhibited by the presence of PPF in the culture medium. Protoplasts and cells from eight non-host plant species were significantly less sensitive than pigeonpea to PPF but cells of chickpea were equally sensitive. The results are discussed with respect to the advantages of the cell assay compared to other assays, the nature of PPF compared to other Phytophthora toxins and its potential as a selection tool in vitro.