A total of 17 children, aged 1 to 15 years, with gingivostomatitis were investigated to follow the development of immune parameters in those who suffered from herpes simplex virus stomatitis. Mouth swabs were obtained during the acute attack. Blood samples were collected on this occasion and again about 3 weeks later. Humoral immunity to herpes simplex virus was investigated by a complement fixation test and by an antibody-dependent cell-mediated cytotoxicity test. Cell-mediated immunity was investigated in a blast transformation assay with herpes simplex virus type 1 antigen and phytohemagglutinin. Interferon production in herpes-stimulated cultures was measured. Thirteen patients had a herpes simplex stomatitis. Twelve of these children were negative in the complement fixation test on the first serum specimen, but only five were negative in the antibody-dependent cell-mediated cytotoxicity test. These five were still febrile at the time of investigation. Blast transformation was negative at the first investigation in most children, whereas interferon was produced both in leukocyte cultures obtained during the infection and also in cultures made 3 to 4 weeks after the infection. An increase in immune parameters was seen in all patients with herpes stomatitis. From results in blast transformation and antibody-dependent cell-mediated cytotoxicity, it is seen that cell-mediated and humoral immunity can be found at the same time during recovery from this type of infection.