Cold agglutinins (CA) are observed during acute infections as transiently occurring postinfection autoantibodies directed against erythrocyte antigens. Distinct infectious agents induce CA of distinct specificity. A common association is the infection with Mycoplasma pneumoniae and the production of CA with anti-I specificity. In infections with varicella-zoster virus (VZV) and rubella virus (RV) a few cases of CA appearance are reported, all of them showing specificity for the Pr antigens. We examined the frequency of CA with the anti-Pr specificity in patients suffering from acute infections with VZV and RV (table 1). The known association of anti-I in about 70% of the cases with acute Mycoplasma pneumoniae infection is confirmed by our results. The occurrence of CA in infections with VZV and RV is a rare event. Based on published data, the specificity of these CA apparently is anti-Pr. Three of 5 anti-Pr examples were found in newborns with rubella embryopathy. Possibly, anti-Pr can be observed more frequently in these patients compared to adults.