A general feature of temperature-induced reversible denaturation of sm
all globular proteins is its all-or-none character. This strong cooper
ativity leads to think that protein molecules, possessing only two acc
essible thermodynamic states, the native and the denatured one, resemb
le 'crystal molecules' that melt at raising temperature. An analysis,
grounded on mean field theory, allows to conclude that the two-state t
ransition is a first-order phase transition. The implication of this c
onclusion are briefly discussed.