Several protease negative mutant strains including HM114, HM126, and HM130 as well as their parent strain KS272 were compared for their growth and secretory production of a model fusion protein, protein A-β-lactamase. HM114, a strain deficient in two cell envelope proteases, grew slightly faster and produced more fusion protein than the other strains deficient in more proteases. HM114 was grown to a cell dry weight of 47.86 g/L in 29 h using pH-stat, fed-batch cultivation. The β-lactamase activity was 11.25 x 104 U/L, which was 30% higher than that obtained with its parent strain KS272. Up to 96% of protein A-β-lactamase fusion protein could be recovered by a simple cold osmotic shock method. The specific β-lactamase activity obtained with HM114 after fractionation was 4.5 times higher than that obtained with KS272.