Cefixime was assayed by a microbiological method in the tonsils of 21 children (mean age 59 months). Tonsillectomy was performed 5 hours after a third dose of 4 mg/kg administered 12-hourly. Plasma cefixime levels were evaluated 10 hours after the second dose with a mean value of 0.84 mg/l (range: 0 to 1.35) and again after a third dose during amygdalectomy with a mean value of 1.24 mg/l (range: 0.1 to 3.9). Cefixime concentrations were 0.74 micrograms/g in the right tonsils and 0.53 micrograms/g in the left tonsils. The antibiotics could not be detected in both tonsils in 6 children and in one of the two tonsils in 11 children. The tissue penetration of cefixime in tonsils therefore was about 1 micrograms/g in those cases where cefixime was detectable. As with other beta-lactam antibiotics, this penetration is not regular, being dependent on the degree of tonsillar fibrosis inhibitin their diffusion.