Extracts of developmental stages and 4 adult organs of the frog, Rana pipiens, were examined by paper electrophoresis. Seven protein cation-bands and 2 protein anion-bands stained with brom phenol blue were detected in extracts of developmental stages from the unfertilized egg through stage 15. By stage 21, 2 of the cation-bands were no longer detected. Examination of the adult organ extracts revealed organ-specific proteins in addition to proteins common to all organs tested. The results were interpreted in terms of the hypothesis that differentiation involves, in part, the loss of ability to synthesize a particular protein(s) and/or changes in the solubility of pre-existing proteins.