In this study, a total of 21 coccoid forms of lactic acid bacteria (lactococci) were isolated from Turkish kefir samples. As a result of the identification tests, 21 lactococci isolates were identified as Lactococcus cremoris (I I strains), Lactococcus lactis (4 strains), Streptococcus thermophilus (3 strains) and Streptococcus durans (3 strains). The amount of produced lactic acid, hydrogen peroxide, proteolytic activity, diacetyl and acetaldehyde productions of the lactococci were determined. Different amounts of lactic acid were produced by strains studies; however, lactic acid levels were 2.3-9.9 mg/ml. While Lac. lactis Z13S, Str. durans Z7S, Z8S, Z15S, Lac. cremoris Z9S, Z16S, Z17S, Z19S, Z20S and Z21S strains were not shown hydrogen peroxide, Lac. lactis Z1S and Z2S strains had a maximum hydrogen peroxide (0.17 mug/ml). Lac. lactis Z2S, Z3S, Str. thermophilus Z5S, Z12S, Str. durans Z7S, Z8S, Lac. cremoris Z14S and Z16S strains were not show proteolytic activity, Lac. cremoris Z20S strain produced the maximum amount (0.09mg/ml) of proteolytic activity. Acetaldehyde concentration produced in Lactobacillus strains ranged between 0.18 and 3.96 mug/ml. Antimicrobial effects of the lactococci on Escherichia coli NRLL B-704, Staphylococcus aureus 4-63, Pseudomonas aeruginosa ATCC 29212 were also determined by an agar diffusion method. All of the strains were able to inhibit S. aureus, while Lac. lactis Z1S, Z2S and Lac. cremoris Z6S strains were able to inhibit E coli and P. aeruginosa. Also, Str. thermophilus Z5S strain were able to inhibit P. aeruginosa. (C) 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.