Various characteristics of membrane and cytoplasmic proteins from human, monkey and other animal lenses have been studied. The polypeptide patterns of lens fiber membranes of man and monkey are very similar. They contain two major polypeptides: a main intrinsic polypeptide of 27 dalton (MIP) and its 22000–23000 dalton conversion product, which can both be extracted by chloroform-methanol. Monkey MIP is immunologically identical to human MIP. Both show a reaction of partial identity with the MIPs of calf, sheep and pig. A weak cross-reactivity is found between primate MIP and chicken MIP. EDTA-extractable proteins (EEP) of 3200 and 35000 dalton can be isolated from monkey lens membrans with low recovery, while EDTA fails to extract any EEP from human lens membranes. The individual crystallins of man and monkey are immunologically closely related. They show reaction of partial identity with the crystallins of calf, sheep and pig. Chicken α - and β -crystallin are partially identical to α - and β -crystallin from mammalian and primate lenses.