The major disadvantage of using alginate-immobilized Aspergillus ochraceus for 11a-hydroxylation of progesterone is its instability in phosphate solutions. In this study, different gel-hardening methods were used to improve the stability of alginate gels. Addition of Fe3+ and Fe2+ ions increased the activity of hydroxylation, but Fe2+ was not a good gelling agent. Replacement of CaCl2 by FeCl3 as the gelling agent increased the activity of the immobilized cells by about 33%. Hardening the alginate-immobilized cells with polyacrylamide (PAA) increased their stability in a 0.1 M phosphate buffer from 0.35 h to over 36 h. The productivity was increased by a factor of 1.8 compared to those without hardening, and the maximum yield was also increased from 84.3% to 90.8%. The physical properties and biocatalytic characteristics of PAA-hardened, immobilized cells were found to be feasible for application in a steroid bioconversion system.