Meropenem and comparator antibiotics, including ceftriaxone, ceftazidime, benzyl penicillin and a combination of ampicillin plus gentamicin, were evaluated in a model of bacterial meningitis in the guinea-pig. The model is an acute infection in which challenge with each organism, if untreated, causes an increase in numbers of white blood cells, elevation of protein concentrations and 6-8 log10 cfu/mL of bacteria in the CSF. Infections caused by Haemophilus influenzae, Neisseria meningitidis, three strains of Streptococcus pneumoniae (two penicillin-resistant), Escherichia coli, Pseudomonas aeruginosa and Listeria monocytogenes all responded to meropenem, which was as active as the comparator agents in all studies, and was more active in most. Of particular note were the results seen against S. pneumoniae (penicillin-resistant) infections, in which meropenem was significantly more effective than ceftriaxone. Also notable were results from the P. aeruginosa infection where meropenem, at low doses, was more effective than ceftazidime. Activity against L. monocytogenes was equivalent to that produced by treatment with the combination of ampicillin plus gentamicin, even when treatment was delayed. These results show that, in an animal model, meropenem penetrates into CSF in concentrations sufficient to produce significant reductions in the numbers of common and less common pathogens.