Selective biocatalyzed synthesis of 2-deoxyadenosine from 2'-deoxypyrimidine nucleosides was carried out using free or immobilized whole cells. The reaction was performed at 57 degrees C without secondary reactions. Two psychrotrophic microorganisms, Bacillus psychrosaccharolyticus and Psychrobacter immobilis, are described for the first time as active and specific strains for the synthesis of 2'-deoxyadenosine. Adenosine deaminase activity was not detected. Whole cells were immobilized in different matrixes. Calcium alginate and calcium pectate gave the best biocatalysts. The synthesis of 2'-deoxyadenosine follows an apparent first order kinetic expression. External mass transfer control was negligible as deduced from k(s), N-A, and Omega values. Internal mass transfer was the rate controlling step according to eta(T) and phi values.