Pacific herring Clupea pallasi immunoglobulin is an IgM-like molecule comprised of heavy and light chains with molecular weights of 79 and 25 to 27 kD, respectively. Purified immunoglobulin was used to generate highly specific polyclonal antibodies for development of a sandwich ELISA. The ELISA was used to quantify total plasma IgM in 602 Pacific herring captured in Prince William Sound and Sitka Sound, Alaska, USA. Plasma IgM concentrations ranged from 0.13 to 5.32 mg ml-1. Using multiple stepwise regression analysis, plasma IgM was highly correlated (p