A method for the preparation of stable suspensions of erythrocytes sensitized with herpes simplex virus (HSV) antigen and for their use in the passive haemagglutination test (PHAT) was developed. Formolized sheep erythrocytes were treated with tannin and sensitized with HSV antigen prepared from infected chick embryo cell culture by ultrasonication and virus extraction with alkaline glycine buffer. Antibody titres determined in the PHAT were higher than titres of neutralizing antibody. The specificity of the results was checked by the passive haemagglutination-inhibition test (PHAIT). The sensitized erythrocytes retained their activity for 5 months (the observation period) and gave reproducible results. The availability of stable erythrocyte diagnostic preparations simplifies the detection of herpesvirus antibody and makes the method widely applicable.