BL-S 578 (Cefadroxil) is a new orally active semisynthetic cephalosporin antibiotic with broad-spectrum antibacterial activity. The new compound was evaluated in vitro in comparison with cephalexin. Some properties studies such as, antibacterial activity, binding with serum proteins and stability in acid and neutral solution at 37 degrees C for both cephalosporins were similar. In experimental infections of mice, the protective action of BL-S 578 was more effective than cephalexin against Staphylococcus aureus and Streptococcus pneumoniae. BL-S 578 was more resistant than cephalexin to the beta-lactamases produced by Klebsiella pneumoniae and Escherichia coli.