This study was undertaken in order to investigate the turn-over of newly synthesized sea urchin embryonic proteins, the distribution of these proteins in the cell and their changes in solubility with time. Refined immunological methods providing unique possibilities for studies of individual proteins were utilized. The proteins were obtained in two soluble fractions: a concentrate of proteins from the cytosol was obtained by homogenization in a hypotonic medium and another fraction constituting membrane-associated proteins was solubilized by detergent treatment of the residual pellet; 75% of the total protein content in early and 60% in later developmental stages was thus solubilized. The proteins solubilized by these mild methods retained their antigenicity and could be analyzed by immunodiffusion methods. The fate of newly synthesized embryonic proteins was studied by incorporation of a radioactive amino acid using two different incubation procedures. The results of these studies indicated that newly synthesized proteins changed their solubility properties with time. Proteins were thus transfered from the soluble to the insoluble fraction. The findings that more proteins became insoluble during development mentioned above also supports such an interpretation. The synthesis of individual proteins was studied by immunodiffusion and autoradiography. Most of the antigens in the two soluble fractions were identical. Experiments with absorbed antisera revealed that only two to four antigens were present exclusively in one of the two soluble fractions. Three groups of antigens differing in regard to their metabolic activities were distinguished. Some antigens were labeled after both procedures without substantially increasing in concentration during development. Thus, their labeling was probably sustained by turn-over. Other antigens were labeled only after one of the two incubation procedures indicating large variations in regard to synthesizing rates of individual proteins.