Differently charged isomers of human pituitary prolactin were subjected to stability studies under various conditions. The study was guided by analytical electrophoresis and radioimmunoassay. In alkaline medium (pH 10) it was found that each isohormone was converted into faster-migrating components, probably due to deamidation. A quantitative study of this alteration was made by measuring the rate constant for this conversion assuming first-order kinetics. The result indicated that the rate constant decreased with increased acidity of the examined prolactin components. Furthermore, this alteration was also found to be paralleled by a decreased immunoactivity of the components. At neutral pH the conversion reaction was studied both in 0.9% NaCl and in human serum. For the least acidic isohormone the rate constant in serum was calculated to be about four times higher than that in the saline solution which in turn was comparatively low. It was concluded that the alteration observed at pH 10 might be attributed to non-enzymic deamidation. The conversion of prolactin that occurred in human serum indicated that this reaction could be caused by enzymes. A study of the immunoactivity of human prolactin as a function of time at various storage conditions is also included in this work. The stability of the hormone was found to be strictly concentration dependent and also affected by buffer ionic strength. In the presence of ethylene glycol (50%, v/v) at -20 degrees C and at a protein concentration in the range of 0.1-2 mg/ml the hormone was found to be both immunologically and electrophoretically stable for several years.