The effects of temperature, dilution, dialysis and the presence of heparin on the stability of lipoprotein lipase (LPL) in milk, skim-milk, milk serum and casein micelles were investigated. At 4 and 20 °C milk serum was the source of the least stable LPL and casein was that of the most stable. There was little difference between LPL stability in milk and skim-milk at these temperatures, or between serum and casein LPL at 50 °C. Heparin (5 µg/ml) increased stability although the effect was less for casein LPL than for serum LPL. A 40-fold dilution of serum LPL with either simulated milk ultrafiltrate (SMUF) or 0·01 M-Tris-Cl pH 8·3 increased the loss of serum LPL, but not of casein LPL. Dialysis of skim-milk against deionized water or SMUF increased stability at 4 or 20 °C but not at 37 °C. LPL activity was more stable in diluted samples of dialysed skim-milk than in diluted samples of the same milk which had not been dialysed. Dialysis against deionized water increased lipolysis but against SMUF it did not increase. Solutions prepared by dialysing water against some milks were found to inhibit lipolysis and this effect was overcome by heparin. The possibility that milk serum contains a factor which influences LPL stability is discussed.