Heavy coagulum formation in lactic casein manufacture results in high casein concentration, high starter bacteria concentration and low pH on the base of the coagulation vessel. No heavy coagulum was formed during slow coagulation of skim milk with glucono-δ-lactone, indicating that starter bacteria were necessary. Skim milk preheat treatments severe enough to cause denaturation of the more heat- stable whey proteins, α-lactalbumin and β-lactoglobulin, also caused a reduction in heavy coagulum formation, suggesting the heat inactivation of a co-factor involved in the process. These results suggest a mechanism for heavy coagulum formation similar to that for minor sludge formation in cottage cheese manufacture. Starter bacteria would clump together, probably assisted by a co-factor. These starter clumps would generate a region of low pH causing isoelectric precipitation of casein in and around the clumps. The clumps would then settle forming a layer on the base of the coagulation vessel which might become further compressed by gravity.