A commercial latex agglutination (LA) kit (Wellcogen, Wellcome Diagnostics) used to detect bacterial polysaccharide antigens (Haemophilus influenzae type b and Streptococcus pneumoniae) was compared with a modified counterimmunoelectrophoresis technique and blood culture for etiologic diagnosis of presumptive bacterial pneumonia requiring hospitalization in 60 infants and children. Serum, urine and either sputum or nasopharyngeal secretions were collected during the first 5 days of therapy for antigen detection. Blood culture was positive in 6 of 52 (11.5%) of cases. Antigens were detected by counterimmunoelectrophoresis and/or LA in 13 of 60 (21.7%) serum samples, 2 of 16 (12.5%) unconcentrated urine samples, 19 of 42 (45.2%) urine samples concentrated 25-fold and 21 of 45 (46.7%) sputum or nasopharyngeal secretions. Antibiotic treatment for 5 days did not affect the antigen detection rate. Counter-immunoelectrophoresis was more sensitive than LA in serum and urine but not in sputum. However, because false positive reactions were frequently obtained with LA on nasopharyngeal secretions of an age-matched control group, this test appears unreliable.