One hundred and sixty-five consecutive patients ( > 2 years of age) with acute group A streptococcal (GAS) pharyngitis randomly received co-amoxyclav (79 patients) or phenoxymethyl penicillin (86 patients). beta-Lactamase activity in saliva was determined for each patient. At follow up after seven days, tonsillar cultures from seven patients (9.6%) in the penicillin V group grew group A streptococcus; three of these patients had tonsillitis clinically. In the co-amoxiclav group these figures were three (3.8%) and two respectively (P > 0.05). Within the 12 month follow up period, there were four clinical recurrences (6.1%) in the penicillin V group and seven (9.3%) in the co-amoxiclav group (P > 0.1). beta-Lactamase activity in the saliva was demonstrated in 29 patients (19.2%). Fourteen (74%) of 19 bacteriological failures or clinical recurrences had beta-lactamase activity, versus 15 (12%) of 129 successfully treated patients (P < 0.001). There is no evidence that oral co-amoxiclav is better than oral penicillin V for the first treatment of acute GAS pharyngitis, but bacteriological failure and clinical recurrence are strongly associated with the presence of beta-lactamase activity in commensal flora.