The relationship between lead speciation and its bioaccumulation by the alga Chlorella vulgaris was studied in the absence and presence of nitrilotriacetic, iminodiacetic, malonic, and citric acids. Pb uptake fluxes were rigorously analyzed by considering the simultaneous effects of metal transport in the medium coupled with metal complex dissociation kinetics. Under the conditions examined here, lead biouptake by C. vulgaris was governed by the free lead ion activity. Potentially labile hydrophilic complexes such as lead citrate and lead malonate did not contribute to the internalization fluxes. Kinetic modeling of the mass transport, adsorption reactions, and internalization fluxes confirmed the rate limiting role of the internalization flux. Comparison of the internalization and diffusive fluxes revealed that even in the presence of a large excess of Pb complexes, the supply of free ion (Pb2+) was sufficient to account for the observed Pb uptake. Pb adsorption to the cell surface was described by Langmuir isotherm. A new method was proposed as a means to estimate the number of Pb occupied transport sites at steady state. The apparent stability constant for the interaction of Pb with transport sites was determined to be 10(5.5) M-1 at pH 6. Low temperature decreased both the Pb uptake flux and the Pb adsorbed to the transport sites. Pb uptake in the presence of Ca was competitively inhibited, and the binding affinity constant for Ca and transport sites was estimated to be 10(4.67) M-1 at pH 6. Results were discussed within the perspective of the free ion activity and biotic ligand models.