Latamoxef (LMOX, Siomarin) at a dose of 2 g was intravenously administered to each of 23 patients undergoing the open heart surgery and the concentrations in serum, pericardial fluid and auricle of heart were measured. Pharmacokinetic observations are summarized below. The peak serum concentration (t = 0) was 227.3 micrograms/ml and the serum half-life (T1/2 beta) was 1.74 hours. In pericardial fluid, LMOX reached the peak concentration of 28.44 micrograms/ml at 4.9 hours and the half-life was 9.99 hours. In auricle of heart, LMOX reached the peak concentration of 42.78 micrograms/g at 6.9 minutes and the half-life was 1.74 hours. It was shown that LMOX penetrates well into the pericardial fluid and the auricle of heart, and it is considered that their levels exceed the minimal inhibitory concentration against a majority of clinical isolates except Pseudomonas aeruginosa.