Gram-negative oral bacteria rapidly produce the odorant hydrogen sulphide from cysteine. It provides a major part of the oral malodour bouquet while causing a corresponding decrease in the oxidation-reduction potential (Eh). A low Eh favours oral putrefaction and malodour occurrence. Challenge testing with cysteine (5ml of 6mM for 30 seconds) enabled evaluation of: the contribution of tongue and teeth bacteria to overall oral malodour; the effectiveness of tongue and tooth brushing, tooth scraping, gum chewing and mouthrinsing with different agents in reducing oral malodour. Successive cysteine challenge tests for 20 minute periods at selected times in a seven hour experiment were effective for assessing the magnitude and duration of an agent's effectiveness. Brushing the teeth reduced malodour modestly. So did tongue scraping and gum chewing. In contrast, brushing the tongue dorsum, especially the posterior half was remarkably effective, which confirmed it as a major site of oral malodour contribution. Rinses containing various actives showed wide variation in effectiveness. The experiments demonstrated that cysteine challenge testing is potentially a aluable tool for assessing the ability of the oral bacteria to produce malodour and for assessing agents designed to inhibit such production.