We identified 3-phenylpropionic acid, citrulline, lactic acid, ornithine, proline, and betaalanine as low-molecular weight substances that are released from cells treated with methylmercury. In this study, we studied their effect on cellular sensitivity to methylmercury. Treating HEK293 and C17.2 cells with each of the six substances minimally affected the proliferation of both cell lines. Among these six substances, however, only citrulline slightly but significantly increased the sensitivity of C17.2 and HEK293 cells to low levels of methylmercury. Citrulline is thought to be a methylmercury toxicity-enhancing factor whose extracellular release is enhanced by methylmercury.