The biotic ligand model (BLM) has the potential to predict biological effects and bioaccumulation in metal mixtures. Pb and Cu uptake by the green alga Chlamydomonas reinhardtii have been quantified in single-metal exposures and in metal mixtures in order to test some of the key assumptions of the BLM. Stability constants for the interaction of the metals with biological uptake sites were determined from measured short-term internalization fluxes. In the absence of competition, a value of 10(5.8) M(-1) was obtained for Cu, while 10(5.9) M(-1) was obtained for Pb. Competition experiments did not show a straightforward antagonistic competition as would be predicted by the BLM. Only at high Cu(2+) concentrations (>1 microM) did Cu behave as a competitive inhibitor of Pb transport. Surprisingly, low concentrations of Cu(2+) had a synergistic effect on Pb uptake. Furthermore, Cu uptake was independent of Pb when Cu concentrations were below 10(-7) M. In order to explain the observed discrepancies with the BLM, membrane permeability and Cu transporter expression levels were probed. The expression of ctr2, a gene coding for a Cu transporter, increased significantly in the presence of Pb, indicating that bioaccumulation is much more dynamic than assumed in the equilibrium models.