Two patients in whom various immunological phenomena obscured and delayed the diagnosis of subacute bacterial endocarditis (SBE) are described. Immunological abnormalities usually correlate well with the chronicity and severity of the disease, as is demonstrated here. The mechanisms underlying the various immunological phenomena are discussed. The importance of alertness to the possibility of SBE in cases of disease manifesting itself as an immunological disorder, thus achieving early diagnosis and early antimicrobial treatment, is stressed.