Mureidomycins (MRD's) A-D were specifically active against Pseudomonas aeruginosa. Among them, MRD C was most active, with MICs of 0.1 to 3.13 μg/ml against many strains of the target organism. Its activity was comparable to that of cefoperazone, ceftazidime and cefsulodin. MRD C-resistant mutants of P. aemginosa appeared spontaneously at a high frequency when cultured in the presence of the antibiotic. No cross-resistance was observed with β-lactam antibiotics. A rapid decrease of turbidity along with spheroplast formation and cell lysis was observed when cells of P. aeruginosa were grown in the presence of MRD C. The compounds exhibited low toxicity and protected mice from experimental infection with P. aeruginosa. The urinary and fecal recoveries of MRD C given subcutaneously were 5 and 18%, respectively.