Yersinia pestis is a gram-negative human pathogen that uses a type III secretion system to deliver virulence factors into human hosts. The delivery is contact-dependent and it has been proposed that polymerization of Yop secretion protein F (YscF) is used to puncture mammalian cell membranes to facilitate delivery of Yersinia outer protein effectors into host cells. To evaluate the potential immunogenicity and protective efficacy of YscF against Y pestis, we used a purified recombinant YscF protein as it potential vaccine candidate in a mouse subcutaneous infection model. YscF was expressed and purified from Escherichia coli by immobilized metalion affinity chromatography and protein identity was confirmed by ion trap mass spectrometry. The recombinant protein was highly alpha-helical and formed relatively stable aggregates under physiological conditions. The properties were consistent with behavior expected for the native YscF, suggesting that the antigen was properly folded. Ten mice were inoculated subcutaneously. administered booster injections after one month, and challenged with 130 LD50, of wild type K pestis CO92. Six animals in the vaccinated group but none in the control group survived the challenge. The vaccinated animals produced high levels of specific antibodies against YscF as determined by Western blot. The data were statistically significant (P= 0.053 by two-tailed Fisher's test ), suggesting that the YscF protein can provide a protective immune response against lethal plague challenge during subcutaneous plague infection. Published by Elsevier Inc.