The performance of 11 single strains of Lactococcus (Lc. lactis ssp. cremoris 223, 227, 250, 255, 267, 303, AM1 and SK11 and Lc. lactis ssp. lactis UC317 [non-bitter strains], and Lc. lactis sap. cremoris Wg2 and HP [bitter strains]) was studied in miniature Cheddar-type model cheeses. Different inocula were used to give equivalent rates of acidification during manufacture and the cheeses made differed in flavour development and proteolysis during ripening. Urea-polyacrylamide gel electrophoretograms (PAGE) of the water-insoluble fraction (WISF) of all cheeses were similar after 2 and 4 months of ripening, while the level of water-soluble nitrogen (WSN; as % of total N) showed only small differences between cheeses made with different starter strains. The cheese made with Lc. lactis ssp. cremoris Wg2 differed considerably from the other cheeses as indicated by urea-PAGE of the water-soluble fraction (WSF), reverse-phase high performance liquid chromatography (RP-HPLC) of the 70 % ethanol-soluble and -insoluble fractions of the WSF, concentration of amino acids and flavour characteristics; it had the highest pH (5.7) and the highest concentration of most of amino acids except Asp, Ile and Arg after 4 months of ripening. The cheese made with Lc. lactis ssp. cremoris SK11 received the highest score for flavour and that made with Wg2 received the lowest score. The results of this study indicate that the strain(s) of Lactococcus used as starter plays an important role in the biochemistry of cheese ripening and produces a characteristic profile of peptides and amino acids. (C) Inra/Elsevier, Paris.