Defence mechanisms against Cu toxicity were examined in two dominant Enteromorpha species from two coastal water types. The macroalgae were collected at three locations in the eulittoral of the Scheldt Estuary (Netherlands, Belgium) and the Thermaikos Gulf (Greece). For 10 days E. prolifera (Scheldt) and E. linza (Thermaikos) were incubated in seawater media of different salinities: 6, 9, 23 psu and 25, 30, 35 psu, respectively. In one series, media were enriched with 100 μg Cu l-1; responses were compared with those in controls with no extra Cu added. Enteromorpha, which is frequently used as a monitor species for heavy metal contamination, had relatively high Cu tissue levels (0.5-3.8 μmol Cu gdwt-1). Cu levels in E. prolifera controls (Scheldt) decreased with salinity; this was not the case with Cu levels in E. linza controls (Thermaikos). During the 10-d incubation algal protein contents and tissue Cu were rather stable. In E. linza (Thermaikos) algal protein contents were significantly lower than those of E. prolifera (Scheldt), although there was no indication for nitrogen limitation in E. linza. E. linza also had much lower glutathione pools than E. prolifera. Only under acute Cu stress (metal addition) did E. prolifera synthesise metal-binding thiols (phytochelatins). Phytochelatin pools are not suitable as an indicator of the Cu levels in these algae. The glutathione redox ratio GSH:(GSH + 0.5GSSG) was used as an indicator of (Cu-induced) oxidative stress. In E. prolifera (Scheldt) this ratio decreased with algal Cu content (P <0.05), from ˜0.5 to ˜0.2. The average glutathione ratios in Enteromorpha from the Scheldt and Thermaikos showed some oxidative stress induction with increasing algal Cu contents, however more clearly if Cu was added. As this redox ratio can also be influenced by environmental factors such as irradiance and desiccation, it may not be useful as an indicator for Cu-induced oxidative stress in situ.