We report on the natural occurrence of human serum antibodies with specificity for a human monoclonal myeloma IgE(DES). These antibodies were of the IgM class, based on their susceptibility to reduction, sedimentation in sucrose gradients, gel filtration and inhibition of agglutination by anti-IgM antiserum. Autoantibody levels were studied in several groups of patients by particle-counting immunoassay using latex particles to which purified monoclonal IgE(DES) was coupled. Only sera of patients suffering from parasitosis had significantly higher levels (p less than 0.0005) than those of healthy blood donors. Cord sera had very low levels, followed by an age-dependent increase during early infancy. There was no relation (p greater than 0.10) between serum IgE and IgM antibody level. On the other hand, significant relations between IgM anti-IgE(DES) levels and serum IgM (p less than 0.0005), serum IgA (p less than 0.001) and serum IgG (p less than 0.05) were observed suggesting that high levels were caused by or related to polyclonal activation of the immune system.