28 pediatric patients with signs and symptoms of infections mononucleosis (MI) are presented. We did a serologic study in all of them from two or three serum specimen collected in 2-3 week intervals. We detected IgG VCA but no antibodies against T. gondii or CMV in all of them. The level of the titers of these antibodies didn't help in the establishment of the phase of the illness. IgM VCA was present in the serum of 22 patients, with or without heterophil antibodies (AH), Meanwhile the remaining 6 patients had no antibodies to IgM VCA or AH. In all serum samples we evaluated the ratio of IgM/IgG EBNA. In patients with primary infection of MI, this value was greater than 1 or equal to 1, meanwhile it was less than 1 in recent or past infections.