Lactobacillus casei CRL 705 was screened, among other meat isolates, for its proteinase and aminopeptidase activities toward synthetic substrates and, according to that, selected for specific assays on muscle proteins. The hydrolytic effects of whole cells, cell free extracts (CFE), and the combination of both on muscle sarcoplasmic and myofibrillar protein extracts was evaluated by SDS-PAGE and reverse phase HPLC analyses. The proteinase activity of whole cells caused the degradation of a great number of sarcoplasmic protein bands. A partial hydrolysis was also associated with CFE that when combined with whole cells showed an important additional degradation. Peptide profiles from sarcoplasmic protein extracts were greatly modified regardless of the addition of whole cells or CFE, although their combination intensified these changes. The generation of free amino acids was remarkable when whole cells and CFE were incorporated together to sarcoplasmic protein extracts.