Counts of aerobic faecal flora were done using serial dilutions of stools cultured on selective media in order to estimate the ecological effects of ceftriaxone. Twenty children were studied (24 courses of treatment), 17 of them with post-chemotherapy agranulocytosis. Enterobacteriaceae disappeared in all cases. After treatment, they rose again, unless another antibiotic was administered. No Pseudomonas could be found, due to the protective isolation. Streptococci D disappeared in 6 children. Staphylococcus aureus was present transiently at a low level in one case. Candida persisted in two cases at the same level in spite of antifungal therapy.