We examined the in vitro cytotoxic effects of an organic arsenic compound contained in seafood, viz. the trimethyl (2-hydroxyethyl)-arsonium cation, or arsenocholine (AsCho), on some murine immune effector cells, such as splenocytes, thymocytes, Peyer's patch lymphocytes, peritoneal macrophages and bone marrow (BM) cells using synthesized pure material. We found that AsCho had no cytotoxicity on most immune effector cells, even at concentrations over 10 mmol dm(-3), and it slightly but significantly enhanced the viability of BM cells at doses over 100 mumol dm(-3). This biological effect of AsCho on BM cells might be direct rather than due to autocrine mechanisms mediating some factors secreted by AsCho-stimulated BM cells, because the culture supernatants of BM cells pre-stimulated with AsCho did not influence the viability of other fresh BM cells. It is interesting that this unique biological effect was found in AsCho, an organic arsenic compound contained in some marine animals that are ingested daily as seafood in many countries. Copyright (C) 2002 John Wiley Sons, Ltd.