ABSTRACT: In order to test the hypothesis that bacterial membrane protein is more slowly degraded than soluble protein in seawater, we examined degradation by natural bacterioplankton of membrane and soluble proteins prepared from the marine bacterium Vibrio alginolyticus radiolabeled with ³H- or ¹⁴C-leucine. First order kinetic constants indicated that proteins in crude membrane extract are degraded at significantly slower rates (1/2 to 1/6) than the soluble proteins. Proteins determined to be intimately associated with the membrane were not degraded during the initial 45 h, while a substantial fraction of soluble proteins was degraded during the same period. The data are consistent with a model in which membrane and cell wall materials severely restrict access of bacterial proteases to membrane proteins. After prolonged incubation, however, membrane proteins started to be degraded, suggesting that proteins protected by membrane components were made available for degradation after ectoenzymatic destruction of membrane components. Our data support the hypothesis that macromolecular organic complexes play a role in temporary storage of dissolved organic matter (DOM) in seawater and that complementary hydrolysis by different ectoenzymes produced by diverse bacterioplankton is important in determining rates and patterns of DOM degradation in the sea.