The growing cells of Serratia marcescens (SM) were immobilized with the interpolymer complex carrier, which is formed by the cationic polymer, poly(allyltrimethyl ammonium chloride-co-acrylamide) [P(TM-co-AAm)], and poly(acrylic acid) (PAA). When the association degree of PAA is suitable to the cationic degree of P(TM-co-AAm), the effective crosslinking network provides the most favorable circumstances for the cell immobilization. The alkaline protease can be produced by the immobilized SM with high activity. Compared with the free cells, the immobilized SM has higher thermal stability, acid-base stability, operational stability, and storage stability. Under the optimum immobilizing conditions, not only the living cells of SM but also thermophilic Bacillus firmus (TBF) were immobilized with the complex of P(TM-co-AAm)/PAA. The results show the carrier of P(TM-co-AAm)/PAA complex to be superior in properties to the usual carriers, such as Na-alginate and carrageenan.