Seven of the nine lectins tested bound to food vacuolar membranes and/or the cytostome of the filter-feeding ciliate Euplotes mutabilis. Strongest binding to the cytostome was found with concanavalin A, which labelled the cytostomes of more than 90% of cells within 10s, and quickly labelled progressively more of the food vacuolar membrane material (80% of cells had labelled food vacuoles after 50s exposure to the lectin). Wheat germ agglutinin specifically labelled the basal halves of a particular group of 8-9 frontal membranelles of every cell within 10s, as well as labelling the cytostome area and food vacuoles of many cells. The specific areas of the cells which bind these lectins are involved in food capture. The frontal membranelles are involved in the interception of particles from the feeding current. The cytostome membrane bears binding sites to lectins of many types; these binding sites may be involved in the selective capture of food particles destined for ingestion, whilst unbound particles may be rejected.