The incidenlal finding of monocional immunoglobuiin components (MC) in some infections prompted us lo study this phenomenon more systematically. Using isoelectric focusing withimmunoblotting {detection limit for MC O'l mg/ml), the following infections were studied for thepresence of MC: visceral leishmaniasis, cytomcgalovirus (CMV) infection, echinococcosis andinfectious mononucleosis. MC were found in 16 of 20 leishmania patients and in eight of 18 CMV patients, but in only one of 20 echinococcosis patients and in none of 30 infectious mononucleosispatients. The MC were mostly transient, where tested. A minority of the MC found in theleishmaniasis patients was shown to bind to leishmania antigens. The specificity ofthe majority ofthe MC remains unknown. Further study is required to explain the high incidence of MC in CMV infection and visceral leishmaniasis.