The concentration of herpes-simplex virus (H.S.V.) in the lesions of adults with recurrent herpes labialis was determined. On the 1st day the vesicle appeared, the fluid within the lesion contained 10(5-3) plaque-forming units (P.F.U.) of H.S.V./mul. By swabbing the surface of the lesions with a sterile pledget, 10(6-2) P.F.U. of virus was isolated from the inflamed labial mucosa. The amount of virus obtained from the labial surface decreased on the 2nd and 3rd day to 10(5-0) and 10(3-0) P.F.U., respectively. In two patients on immunosuppressive drugs, high concentrations of virus (greater than 10(4-0) P.F.U.) were obtained per swab for more than 3 weeks. The presence of infectious virus-antibody (V.A.) complexes in herpetic lesions was demonstrated by examining fifty-two isolates from twenty-eight patients at various times during the course of their disease. 71% of the patients had V.A. complexes in their lesions on the 1st day of the vesicular eruption, and by the 3rd day all of the lesions examined contained complexes. It is concluded that patients with active lesions shed high concentrations of virus and that natural infection may be transmitted by an infectious V.A. complex.