Lactalbumin in acid pH exhibits a time-dependent aggregation, the characteristics of which have been studied using sedimentation and solubility methods. Exposure of the protein to pH values below the region of its isoelectric point yields a component having a sedimentation constant of 10-14 S (infinite dilution) in contrast with the value of 1.93 S obtained for the monomeric molecule. The rate of formation of the heavy component decreases with decreasing temperature and decreasing pH and ionic strength. Similarly, reduction of the net molecular charge through binding of anions such as nitrate dramatically increases the rate of aggregation.