Cefotaxime plus minocycline has been shown to have synergistic activity against Vibrio vulnificus; however, the clinical role of cefazolin in combination with minocycline in immunocompromised hosts has not been established. Therefore, antimicrobial susceptibility of the V. vulnificus clinical isolate Vv05191 was studied by the agar dilution method. Antibacterial activity of cefazolin, minocycline, and a combination of the two drugs was investigated by time-kill studies in vitro and further examined for therapeutic efficacy in a murine model. When cefazolin at a combination of 4 mg/L (1/2 x MIC) was combined with minocycline at a concentration of 0.03 mg/L (1/2 x MIC), sustained inhibitory activity was noted until 24 h. In BALB/cByJ mice with cyclophosphamide-induced neutropenia, an inoculum of 1.5 x 10(8) CFU caused death within 96 h when the infected mice were treated by cefazolin (400 mg/kg every 3 h), while 6.3% of mice survived when treated by minocycline (4 mg/kg stat, then 2 mg/kg every 12 h). However, 62.5% of mice survived for 96 h when mice were treated by cefazolin (400 mg/kg every 3 h) plus minocycline (4 mg/kg stat, then 2 mg/kg every 12 h) (P = 0.002, log rank test). In conclusion, cefazolin in combination with minocycline exhibits in vitro synergistic antibacterial activity against V. vulnificus and provides a therapeutic advantage in neutropenic mice with V. vulnificus infection.