A cassette based on the expression signals of the Lactobacillus brevis surface (S)-layer protein gene (slpA) was constructed. The lour-copy-number vector pKTH2095, derived from pGK12, was used as the cloning vector. The efficiency of slpA promoters in intracellular protein production was studied using three reporter genes, beta-glucuronidase (gusA), luciferase (luc) and aminopeptidase N (pepN) in three different lactic acid bacteria hosts: Lactococcus lactis, Lactobacillus plantarum and Lactobacillus gasseri. The S-layer promoters were recognized in each strain and especially L. lactis and Lb. plantarum exhibited high levels of transcripts. The production kinetics of reporter proteins was studied as a function of growth. The GusA, Luc and PepN activities varied considerably among the lactic acid bacterial strains studied. The highest levels of beta-glucuronidase and luciferase activity were obtained in L. lactis. The level of GusA obtained in L. lactis corresponded to over 15% of the total cellular proteins. The highest level of aminopeptidase N activity was achieved in Lb. plantarum where PepN corresponded up to 28% of the total cellular proteins at the late exponential phase of growth. This level of PepN activity is 30-fold higher than that in Lb. helveticus, which is the species from which the pepN gene originates.