Postmortem concentration of the three immunoglobulins IgA, IgG and IgM has been studied by single radial immunodiffusion in 81 cases of sudden unexpected death in infancy and in 6 deaths between 2 and 11 years of age. The examinations of serum were repeated after several days after the sera had been kept at different temperatures. For comparison the serum immunoglobulin levels of IgA, IgG and IgM were determined in 11 corpses of adults directly after death and again 1 or 2 days later. RESULTS With the radial immunodiffusion method postmortem serum immunglobulins are determinable. A critical estimation of postmortem IgA-, IgG- and IgM serum levels has to consider postmortem protein modifications and keeping sera at higher temperatures (+44 degrees C., + 20 degrees C.) For determinations at a later date sera must be kept at -35 degrees C. The measured postmortem serum levels of IgA and IgG in cases of sudden unexpected death in infancy correspond with the normal variation of value in healthy children of the same age. The lowest concentrations of IgG were found about the 5th. month in infancy. Many of the IgM levels were higher than the normal mean value in healthy children of the same age. This is not caused by postmortem influences. The higher IgM concentration in sera suggest an active immunological reaction before death.