The molecular characteristics of particulate protein in the surface waters along transects from 45N to 25S in the central Pacific are reported. The majority of particulate-combined amino acids (PCAA) was in the form of protein molecules in the samples from the northern North Pacific and Equatorial regions, namely, productive areas, while PCAA was mainly present as nonproteinaceous amino acid in the subtropical regions, namely, oligotrophic areas. Thus, it appears that the chemical form of PCAA, one of the major constituents of particulate organic matter (POM), varies meridionally. Two characteristic groups of particulate protein were identified from meridional differences in their molecular distribution. The first group, derived directly from cellular proteins of living organisms, was made up of a large number of proteins, each present at a relatively low level, which gave smeared electrophoretograms and were considered to be “background” proteins. The background proteins contributed greatly to the total protein, as well as to PCAA, and they appeared to be readily remineralized. The second group included a small number of specific proteins with a limited range of molecular masses. This group was prevalent in oligotrophic areas, an indication that proteins from specific sources survive and accumulate as a consequence of their resistance to degradation. A protein with an apparent molecular mass of 45 kilodaltons (kDa), a member of the second group, was commonly found at low latitudes and the partial N-terminal amino acid sequence indicated that the 45-kDa protein was a single protein species that has not previously been reported. Thus, a single and identifiable protein molecule appears to be very widespread at low latitudes.