17 infants and children with pyogenic meningitis (14 Haemophilus influenzae, 2 Diplococcus pneumoniae, 1 Neisseria meningitidis) were treated with thiamphenicol, 100 mg/kg body weight/day in 4 doses i.v., as single drug. In the H. influenzae group 10 patients were cured, 4 had relapses of meningitis, 3 with documented subdural effusions. This group is compared with 14 children matched for age, initial leucocyte and CSF cell count treated with ampicillin: all of these were cured, 1 had a subdural effusion. Thiamphenicol concentrations were determined in the serum and CSF 2 h after administration. The mean serum levels were between 10-12 mcg/ml, the mean CSF levels varied from 5.4 mcg/ml at the beginning to 1-1.9 mcg/ml at the end of meningitis. The MIC of H. influenzae was 0.6-12 mcg/ml. A significant, acute, and dose related bone marrow toxicity of thiamphenicol could be documented, but was always rapidly fully reversible. We conclude that thiamphenicol cannot replace chloramphenicol in the treatment of pyogenic meningitis as single systemic antibiotic. Special indications for thiamphenicol in this disease are discussed.