Summary The concentration of serum folate in 260 patients with viral and mycoplasmal infections was determined (305 samples). In 60% the serum folate concentrations were found to be below 3 μg/1. The incidence of low serum folate varied slightly. According to the infections diagnosed, low values were: for influenza A 50% (50 patients), influenza B 42 % (45 patients), human parvovirus 67% (76 patients), rubella 62% (13 patients), infectious mononucleosis 6o % (15 patients), Mycoplasma pneumoniae 73% (45 patients). For a group of undiagnosed rashes it was 81 % (16 patients). The cause of low concentrations of serum folate in these patients is discussed.