The ability of microorganisms to selectively adsorb various heavy metal ions has been recognized for over a decade. We have investigated the biosorption of lead by an active culture of the cyanobacterium Anabaena cylindrica. Energy dispersive X-ray microanalysis was used to evaluate the different uptake mechanisms in the various subcellular regions. Three were identified : a very fast adsorption mechanism in the cell envelope; a time-dependent deposition reaction on the cell surface; and an adsorption mechanism, also time dependent, on the polyphosphate body inside the cell. Atomic absorption spectrometry was then used to quantify the changes with time of bulk fluid concentrations of lead solutions exposed to cyanobacteria. A mass transfer kinetic model was developed which quantitatively predicts the concentration of lead in cells of Anabaena cylindrica as a function of spatial dimensions and time. The model predictions are consistent with a pattern, documented in literature and confirmed by our own experimental evidence, of a very fast uptake in the cell envelope and then a longer uptake period inside the cell. Our experimental evidence also revealed a time-dependent uptake mechanism on the surface of the cells, which is included in the model.