The rabbit liver, heart, and muscle extracts showed isoelectric focusing peak patterns of LDH (Ampholine pH 3.5-10), which considerably differed from the LDH zymograms characteristic to those tissues. Ampholine did not give any significant influence on enzymic activity and electrophoretic mobility (agar gel plate) of the five fractions of purified LDH isozymes. Isoelectric focusing of LDH band 1 (H₄) produced an acidic peak but with a very poor recovery of activity (1-2%), while that of LDH band 5 (M₄) gave an alkaline peak with a good recovery of activity (88%). LDH bands 2 (H₃M), 3 (H₂M₂), and 4 (HM₃) exhibited multiple isoelectric focusing peaks, of which an alkaline peak was dominant. Isoelectric focusing of the DANSylated LDH isozyme, band 3, revealed 3 to 4 fluorescent protein peaks not corresponding to the activity peaks. Dissociaton and partial reactivation of LDH (tetramer), accompanying aggregation-denaturation (inactivation), especially in acidic pH, are responsible for producing the modified activity peak pattern of LDH isozymes in the isoelectric focusing experiments.