Abstract Low MW Igs with truncated heavy chains have been described in a small number of reptilian species and anseriform birds. While these shortened Abs can bind Ag, their immunological significance is not understood since they are unable to mediate effector functions performed by full-length Ig. We isolated a low MW form of IgM from chicken serum by gel filtration chromatography. The low MW IgM was present in both adult heparinized plasma and EDTA-plasma containing protease inhibitors, but was not detected in sera of 2-3 week old birds. Thus it is unlikely that low MW IgM was derived from the native pentamer/tetramer by proteolytic fragmentation. Low MW IgM retained biologic activity as a partially purified isolate prepared from anti-DNP antisera exhibited hapten binding as determined by ELISA. SDS-PAGE of serum under non-reducing conditions followed by Western Blotting using a polyclonal goat anti-mu chain Ab revealed two low MWspecies of about 260 and 160KD. Both low MW forms were detected by Western Blotting with a monoclonal anti-mu chain and anti-L chain Abs, but only the larger form was detected with a second anti-mu chain Ab. These observations suggest the possibility that at least the smaller of the low MW IgM species contains a truncated heavy chain.