Conjugation in the ciliate Tetrahymena thermophila has been used as a system in which to analyze biochemical events associated with the execution of a complex cell-cell interaction. Two-dimensional electrophoretic analysis of [35S]methionine-labeled whole-cell proteins revealed major changes in protein synthesis correlated with costimulation and the onset of pairing; specifically, the major induced polypeptide was one of 80 kDa. A second change in the pattern of protein synthesis was associated with the onset of meiosis; the major induced product was another, perhaps related, 80-kDa polypeptide. An effort was made to detect changes in the patterns of membrane proteins and Con A-binding proteins during conjugation; no changes were found. These results are discussed in the context of earlier hypotheses regarding the distribution of Con A receptors on the surfaces of conjugating cells.