Prematuration of cheesemilk involves the addition of cultures of lactic acid bacteria to milk and incubation at temperatures ranging from 8 to 16 degreesC for periods up to 24 h, without extensive acidification. Prematuration temperature, individually or in combination with starter type or time, had a significant effect on pH, rennet clotting time (RCT) and the concentration of free amino acids in milk. However, no major differences were evident between the 2% TCA-insoluble or -soluble fractions of prematured milk as seen by urea-PAGE or by RP-HPLC, respectively. Prematuration starters had little effect on the growth of mesophilic or thermophilic cheesemaking starters or on the rate at which pH decreased during incubation. Furthermore, they did not effect the growth of typical indigenous microflora. Although RCT decreased with decreasing pH, results show that the slight decrease in pH which normally occurs during prematuration would result in only a small decrease in RCT. Little difference was evident between chemically (i.e., 4% lactic acid) and biologically (i.e., prematuration) acidified samples with regard to RCT, indicating that any decrease in RCT observed in this study was caused by the effect of pH alone and not by any biological effect of prematuration.