The nature of the bimodal denaturation endotherm, which has been observed for some alpha-keratin materials, was investigated in order to evaluate the validity of the two conflicting theories to interpret the phenomenon. Ortho-and para-cortical cells were isolated from Merino wool and subjected to measurements by differential scanning calorimetry in water at elevated temperatures (80-170 degrees C). The results show for the isolated cell fractions denaturation peaks at 138 degrees C (ortho) and 144 degrees C (para) that are with respect to their location and temperature difference in good agreement with the results obtained for the whole fiber material. The denaturation enthalpy of the para- (21 J/g) was found to be higher than of the ortho-cortical cells (17 J/g) in contrast to expectations from electron diffraction studies. The fact that both of these values are higher than for the whole fiber (15 J/g) is attributed to the removal of the cuticle prior and to the enzymatic digestion of some further material during cell separation. The results give unequivocal evidence for the validity of the ortho/para-hypothesis for the interpretation of the endothermic denaturation doublet of keratins.