Adsorption of cadmium and lead ions to floc-type biosorbents was reported in this work. Two types of biosorbents containing a marine microalga, Heterosigma akashiwo (Hada) Hada, or a purple non-sulfur bacterium, Rhodobacter sphaeroides, were prepared. The micro-organisms inactivated by steam sterilization were immobilized in casein floc and cross-linked with glutaraldehyde. In the present immobilizing method, we obtained the biosorbents comprising as much as 67% of micro-organism on a dry-weight basis. Simple metal-binding models were applied to explain the adsorption mechanism of bivalent metal ions to the biosorbents. The results showed that casein acted as both the immobilizing material and the adsorbent material. The adsorption of bivalent metal ions to the biosorbents was due to bidentate binding to the acidic sites on casein and monodentate binding to the acidic sites on micro-organisms. The metal-binding constants and the binding capacities of micro-organisms were scarcely influenced by immobilization.