Acute phase and convalescent sera from 51 pediatric patients who had a documented viral infection and no obvious culture-confirmed bacterial infection such as meningitis, otitis media or urinary tract infection were tested by enzyme immunoassay for antibodies to Haemophilus influenzae and Branhamella catarrhalis and by the latex agglutination test for pneumococcal antigens to evaluate the frequency of mixed bacterial and viral infections. A mixed bacterial and viral infection was documented in 19 patients (37%). Seven patients (14%) showed a diagnostic rise in antibodies to H. influenzae and 8 patients (16%) showed an antibody elevation to B. catarrhalis in their paired sera; pneumococcal antigen was detected in acute phase serum from 4 patients (8%). The rate of mixed infections in patients having respiratory symptoms was 52%. High serum C-reactive protein values and white blood cell counts were found significantly more often in those with mixed infections than in those who had viral infections. The results indicate that mixed bacterial and viral infections occur more frequently in children than one could anticipate on the basis of the earlier reports. Mixed bacterial and viral etiology is highly probable in a child who has a defined viral infection with high C-reactive protein and white blood cell count values, especially in the presence of respiratory symptoms.