Cepaea hortensis is a widespread terrestrial pulmonate, contributing significantly to element fluxes in soil ecosystems. Due to its capacity of accumulating certain trace elements in its tissues, Cepaea hortensis can serve as a biological indicator of metal accumulation in contaminated areas. In response to Cd exposure this species and related helicid pulmonates are also able to synthesize an inducible, Cd-binding metallothionein (MT) isoform specifically serving in binding and detoxification of this metal. As shown by field-collected garden-snails from a metal-contaminated site near a zinc smelting works in Avonmouth (UK) and an unpolluted reference site in Reutte (Tyrol, Austria), Cd and Cd MT concentrations in midgut gland of C. hortensis from these sites increased with rising Cd concentrations in the soil substrate from the same contaminated sites. By combining the results of these field data with laboratory experiments it appears that midgut gland Cd-MT of Cepaea hortensis seems to fulfil the criteria of a successful biomarker in many respects. First, the synthesis of the protein can rapidly be induced by Cd exposure. Second, the level of Cd MT induction in C. hortensis directly reflects the intensity of metal exposure. Third, the induced signal of increased Cd-MT concentration in C. hortensis is persistent over extended periods of time. Fourth, the Cd-MT signal in C. hortensis seems to be very specific for Cd exposure. Regression analyses demonstrate that tissue levels of Cd and Cd MT in C. hortensis depend on Cd concentrations in the substrate which is represented by either soil or plant material on which snails normally feed. In both cases the best fit for this dependence is exhibited by a semi-logarithmic relationship, with substrate (soil or plant feed) concentrations expressed on a logarithmic scale. It is concluded that C. hortensis and other related pulmonates can successfully be used either as biological indicators of Cd accumulation, or as key species in biomonitoring studies focusing on Cd-MT induction as a biomarker for Cd exposure.