In 82 patients with acute tonsillitis studied, beta-hemolytic group A streptococci were isolated from 30 (37%), and group C or G streptococci from 12 (15%). In the 40 patients with non-streptococcal tonsillitis there was a significantly higher isolation rate of pneumococci, H. influenzae and/or B. catarrhalis, as compared with those with beta-hemolytic streptococci. Patients were classified regarding clinical status according to standardized criteria as severe, moderate, or mild. The patients with group A streptococcal tonsillitis were significantly more often classified clinically as 'severe' and had significantly shorter duration of symptoms before seeking medical care, as compared with those with non-streptococcal tonsillitis. Significant increases in white blood cell count and in anti-DNase B were found in the patients with group A streptococcal tonsillitis, whereas their antistreptolysin O levels did not increase significantly. C-reactive protein concentrations were consistently higher in the patients with group A streptococcal tonsillitis. No evidence of polyclonal beta-lymphocyte stimulation was found when measuring antibodies against pneumococci and group B streptococci. The findings show clinical and simple laboratory tests to be useful aids in distinguishing group A streptococcal tonsillitis from non-streptococcal tonsillitis, and that other bacteria may be involved in non-streptococcal tonsillitis.