AbstractWhole cells, mureinoplasts, and protoplasts of gram‐negative bacteria were used in cadmium biosorption and toxicity determinations. It was shown that whole cells ofPseudomonas fluorescensbind less metal than other bacterial forms, suggesting the outer membrane acts as a barrier toward metals. Such a role was confirmed by toxicity measurements showing a lower toxic effect of cadmium in the presence of the outer membrane. Chemically modified bacteria, with carboxyl groups masked, were used to identify the cadmium binding sites. Results indicate that care is necessary in interpretation since the acid pH used in the blockage reaction was shown to interfere with the cadmium binding. Moreover, the blockage reagents have been shown to modify the cadmium toxicit