Rainbow trout, Oncorhynchus mykiss, is one of the cold water fishes cultured in our country, and the intensification of culture of rainbow trout led to outbreak of several diseases which made the immune system study a new research field.In this study, the truncated heavy chain of IgM was obtained by RT-PCR from rainbow trout head-kidney RNA extracts and cloned into pET-27b vector.The predicted molecular weight of recombinant IgM was approximately 47.7 ku.After purification, the purity of recombinant IgM reached approximately 90% confirmed by SDS-PAGE and HPLC analyses.The rabbit antisera against recombinant truncated IgM in this study could react specifically with both natural IgM from rainbow trout serum and the purified IgM in ELISA test.The titer of rabbit antisera against the recombinant truncated IgM was 1:40 000, the natural IgM was 1:20 000 in an antiserum dose-dependent manner.These results indicated that the recombinant IgM expressed in this study had the comparable structure with the natural IgM of the rainbow trout serum.The results provide important experimental basis for trout immune system study and related immunodetection of rainbow trout.