A modified, indirect immunofluorescent technique for the detection of specific serum Herpes simplex virus IgM antibodies is described. The previously necessary long incubation period with the serum for the determination of the IgM fluorescent antibody titre, is reduced from 3 to 1 hour after staphylococcal adsorption of IgG. This IgM test will also, in contrast to the previous procedure with untreated sera, give a more reliable and easy determination of the IgM titres, because the IgM fluorescence is more intense. Paired sera from 34 patients, taken approximately 6 and 15 days after the onset of symptoms, were examined. Sera from 11 patients showed a fourfold or greater rise in titre in the CFT and the IgG fluorescent antibody test, and 7 of these showed also a fourfold or greater rise in the IgM fluorescent antibody titre. The sera from the other patients, however, showed a constant titre in the CFT and the IgG fluorescent antibody test and were negative in the IgM test. The results show that in many cases a current Herpes simplex infection rapidly can be identified by this immunofluorescent technique.