Cefodizime is one of the new broad-spectrum cephalosporins. It is an aminothiazolyl iminomethoxy cephalosporin which is metabolically stable and has a prolonged serum half life. Cefodizime was primarily active against gram-negative bacilli and at the concentration of 0.5 mg/L, it inhibited 90 per cent of Enterobacteriaceae. P. mirabilis was the most susceptible species tested (MIC90 of 0.02 mg/L). E.coli, K.pneumoniae, Salmonella spp., Shigella spp. and M. morganii were also very sensitive to cefodizime, with the MIC90 of 0.25-0.5 mg/L. Cefodizime, however, was not active against most gram-negative bacilli possessing Type I beta-lactamases of Richmond and Sykes, namely, Enterobacter spp., P. aeruginosa and A. anitratus (MIC90 of > 128 mg/L). Among gram-positive bacteria, only S.pyogenes was highly susceptible (MIC90 of 0.05 mg/L), while S. aureus (methicillin-sensitive) was moderately susceptible and Enterococcus spp. was resistant. Cefodizime appeared to be bactericidal and was not affected by serum. High inoculum (10(7) cfu/ml) of K.pneumoniae and Enterobacter spp. resulted in increase of the MIC of cefodizime. This study shows that local bacterial isolates in a university hospital in Bangkok, Thailand were not different in susceptibility pattern from those reported in developed countries. The in vitro activity of cefodizime as a third generation cephalosporin, with its good pharmacokinetic property, and the property of the agent as a biological response modifier, should prove that this is a promising new agent in treating serious infections especially in immunosuppressed hosts.