Double-label immunocytochemistry reveals that immunoreactivity for two putative peptide transmitters, egg-laying hormone and alpha-bag cell peptide, co-exist in most bag cell somata and processes in the abdominal ganglion of the marine mollusc Aplysia. Together with previous physiological and biochemical data these findings indicate that the neuroendocrine bag cells are a multitransmitter system which utilizes two or more peptides derived from a common precursor.