Patients diagnosed as suffering from erysipelas or cellulitis were subjected to bacteriological and serological investigations. The serological tests used included the anti-streptolysin O reaction (ASO), the anti-deoxyribonuclease B test (ADB) and the anti-hyaluronidase tests (AHT) that are specific both for the group A streptococcus (Streptococcus pyogenes) and for the human pyogenic streptococci of group C or group G. Antibody tests to the alpha-lysin and the nuclease of Staphylococcus aureus were also employed. Conventional bacteriological culture methods were used plus needle aspiration of injected saline in most patients with erysipelas, but recognized pathogens were isolated in only 42% of cases. Our results indicate the limitations of these tests for making initial diagnoses and deciding treatment. Serial serological testing was very successful in differentiating cellulitis due to group A, C or G haemolytic streptococci, or occasionally Staphylococcus aureus, but was positive in only 40% of cases of erysipelas.