Cimetidine is widely prescribed as a palliative or cure for gastric disorders in man. It is known to be noncarcinogenic to rodents and has been shown to be inactive in a wide range of in vitro and in vivo genotoxicity assays. It was therefore of concern when an injectable formulation of this drug (Cimetex) was reported to initiate UDS in cultured primary rat hepatocytes [Martelli et al, 1983]. We have confirmed the ability of Cimetex to elicit UDS in primary hepatocytes and established that cimetidine itself is inactive. These data indicate that it is not cimetidine per se, but rather its protonated formulation which is responsible for the activity. These observations pose fundamental questions regarding the validity of the UDS end-point when testing salts in vitro, rather than presenting a challenge to the nongenotoxic status of cimetidine.