The sensitivity to fosfomycin which is found in organisms that are frequently found in urinary infections and the lack of toxicity of this antibiotic were the reasons for which we tested fosfomycin on patients who were subjected to periodic hemodialysis. We were interested in studying (1) if fosfomycin was dialyzable, and (2) its therapeutic pattern in these patients. We selected a group of 27 patients from our programme of periodic hemodialysis who spend 18 h each week in three sessions with an RSP Travenol artificial kidney, using Ultra Flo II as the dialytic unit. They were all free of infection and had not received any antibiotics for 40 days before the test was carried out. They were administered 1 or 2 g of fosfomycin by the intravenous route at the time of beginning the dialysis. Blood samples and samples from the dialytic bath were taken before administering the fosfomycin, and ½, 3 and 6 hours after administering it. Our results show that fosfomycin is 70–80% dialyzed by the membranes of the artificial kidney and that during the hemodialysis, given the clearance which the di-alyzing membrane makes of the antibiotic, the usual doses of fosfomycin should not be altered. No side effects were observed in any of the patien