Most material used for control and calibration in a clinical laboratory is based on pool sera of human origin, guaranteed to be HBsAg-free. Since little information is available on the potential infectivity of HIV, the causative agent of acquired immunodeficiency syndrome (AIDS), 54 control and calibration sera, in routine use, were investigated for the incidence of antibodies to HIV by means of Elisa. Sixteen test specimens ( = 30%) gave positive or borderline Elisa results and were further analysed by immunoblotting, resulting in 15 samples all recognizing gp 160 and partially the p 24, p 31, p 55, p 64 and gp 120 band. Only one sample with borderline Elisa result was negative by this assay. Furthermore, all sera were examined for the presence of viral antigen by a solid phase Elisa. All samples under investigation gave negative antigen Elisa results. Bearing in mind that the sensitivity of this assay is limited to 10 micrograms/l of viral antigen, no conclusion on infectivity should be drawn. The high incidence of HIV-antibodies in the sera investigated demands that this material should be handled with special care by laboratory personnel.