Fourteen vats of Manchego cheese were manufactured from uncooled raw sheep milk; 7 experimental vats inoculated with 1% Streptococcus lactis culture in skim milk and 7 non-inoculated control vats were simultaneously manufactured. Experimental cheeses showed significantly lower pH values than control cheeses throughout the curing period, average differences exceeding 0.2 pH unit at all stages of ripening. Mean log counts of vat milks were 5.90 for Enterobacteriaceae, 5.64 for coliforms and 4.00 for fecal coliforms. After 60 d, log counts of these three microbial groups in experimental cheeses averaged 1.83, 1.46 and 1.02, respectively, while their mean values in control cheeses were 3.78, 3.60 and 2.64; differences between experimental and control cheeses were all significant at the 5% level. Enterobacter cloacae, Escherichia coli and Hafnia alvei were the only Enterobacteriaceae species detected in 60-d-old cheeses. Sensory evaluation data, salt and water content of experimental and control cheeses showed no significant differences.