Raw, pasteurized, and sterilized skimmilk samples from the same lot were analyzed for ionized calcium content by a resin method and by a liquid ion-exchange method. The resin IR 120 was used in the cation exchange method of Christianson et al. (6), while an Orio calcium activity electrode was used for the liquid ion-exchange method. The three milk types were analyzed after four days storage at 4C.An analysis of variance of the data showed that there was no significant difference between the results obtained for the raw, pasteurized, and sterilized milk by the two methods. Thus, it was concluded that the calcium activity electrode provides an accurate method for determination of the concentration of ionized calcium in skimmilk. The calcium electrode method has the advantage of speed, ease of analysis, and repeatability. The results also indicated that sterilization at 150C for 5sec had little or no effect on the concentration of ionized calcium, whereas pasteurization at 65C for 30min caused a significant decrease. The decrease in ionized calcium caused by pasteurization still was pronounced after four days of storage at 4C.