We have measured dissolved protein at a variety of depths at three stations in the Pacific, ranging from the tropics to the subarctic. Most of the dissolved protein at these stations is distributed over a wide range of molecular masses, but consists of fewer than thirty individual proteins. One, with an apparent molecular mass of 48 kDa, is a major constituent at all stations. Its N-terminal amino acid sequence was found to be a homologue of porin P, a trans-outer-membrane channel protein of Gram-negative bacteria. Correspondence of N-terminal amino acid sequences and apparent molecular masses between this dissolved protein and porin P indicates that almost the complete homologue of porin P, from the N-terminus to (probably) the C-terminus, survives without modification in the water column. Persistence of appreciable amounts of an identifiable protein suggests a pathway for production of dissolved organic matter whereby enzyme-resistant biopolymers survive and accumulate in the sea.