The predatory ciliate Lembadion bullinum releases a chemical compound (L-factor) which induces morphological changes in another ciliate, Euplotes octocarinatus, and some of its relatives. The changes render Euplotes so extended that Lembadion has difficulties engulfing this prey. We have previously shown that the L-factor is a protein with a mass of 31.5 kDa. Here we report the primary structure of the L-factor deduced from cDNA, the heterologous expression of the Lembadion gene encoding the L-factor, the production of an antibody directed against this factor and the labeling of the cell surface of Lembadion with this antibody. From this and from peculiarities in the amino acid composition of the L-factor we conclude that the L-factor is a surface protein of Lembadion or at least a major part of it. To our knowledge the L-factor is the first kairomone whose origin and structure has been identified.