Cell viability, autolysis and lipolysis were studied in Cheddar cheese made using Lactococcus lactis subsp. cremoris AM2 or Lactococcus lactis subsp. cremoris HP. Cheddar cheese was made in triplicate over a 3 month period and ripened for 238 days at 8 degreesC. Cell viability in cheese was lower for AM2 (a non-bitter strain) than for strain HP (a bitter strain). Autolysis, monitored by the level of the intracellular marker enzyme, lactate dehydrogenase (EC 1.1.1.27) in cheese 'juice' extracted by hydraulic pressure, was much greater in the cheese made using AM2 than that made with HP. Lipolysis was determined by the increase during ripening of individual free fatty acids (FFA) from butyric (C-4:0) to linolenic acid (C-18:3) measured using a high performance liquid chromatographic technique. Levels of individual FFA from butyric (C-4:0) to linolenic (C-18:3) acids increased significantly (P<0.05) during ripening in cheeses made with either starter culture. Palmitic (C-16:0) and oleic (C-18:1) acids were the most abundant FFA throughout ripening in all cheeses. Levels of caprylic (C-8:0), myristic (C-14:0), palmitic (C-16:0) and stearic (C-18:0) acids were significantly higher (P<0.05) in cheeses manufactured with Lc. lactis subsp. cremoris AM2 than in cheeses manufactured with Lc. lactis subsp. cremoris HP. Differences in levels of lipolysis between strains was not due to differences in the specific lipolytic or esterolytic activities in cell free extracts of the strains as measured by activity on triolein (lipase) and p-nitrophenylbutyrate (esterase) substrates. Therefore, evidence is provided for a relationship between the extent of starter cell autolysis and the level of lipolysis during Cheddar cheese ripening.