Sera from 31 human immunodeficiency virus (HIV)-infected patients, representing different clinical stages of HIV infection, were assayed for antibodies against measles and mumps viruses by various serological tests and compared to 23 healthy controls. Sera from four patients (two primary, one asymptomatic, and one acquired immunodeficiency syndrome) exhibited a pronounced antibody response to measles as detected by haemagglutination inhibition and radioimmuno-precipitation assay. The RIPA-positive sera showed increased reactivity to all the viral components and in particular to the haemagglutinin (HA) protein of the virus (Fig. 1). Three of these positive patients also showed a similar response to mumps virus. One of the control sera also showed an increase in antibody titre in measles serological tests. The measles antibodies were shown not be anti-HIV antibodies crossreacting with paramyxoviruses. The reactivity to haemagglutinin was still present when using nonglycosylated measles virus antigen grown in the presence of tunicamycin. Whether the accentuated antibody response is due to polyclonal activation mediated by HIV or to reactivation of the viruses remains to be answered.