A group of nine well-nourished patients, with normal serum vitamin K1 levels (mean 546, range 310-1350 pg/ml), maintained normal prothrombin times (PTs) and factor VII clotting activities throughout a 7 d course of i.v. cefotetan disodium, an N-methyl-thiotetrazole (NMTT) containing cephalosporin antibiotic. However, 11 of 20 patients, with acute intra-abdominal sepsis and initially normal PTs who underwent emergency surgery, developed prolonged PTs (INR 1.4-3.1) associated with reduction in factor VII activities (0.74-0.38 u/ml) after 3-7 d of antibiotic therapy. Nine of these 11 patients had clinical evidence of malnutrition and nine had subnormal serum vitamin K1 levels (mean 119, range 43-354 pg/ml) on admission. Seven received cefotetan but four were treated with a non-NMTT-containing cephalosporin or antibiotics belonging to other groups. The nine patients who maintained normal PTs all had normal nutritional status and normal serum vitamin K1 levels (mean 279, range 103-915 pg/ml) at presentation. The PT is a relatively insensitive indicator of vitamin K stores, and malnourished patients with low serum vitamin K1 levels are at risk of developing hypoprothrombinaemia following intravenous antibiotic therapy.