The intent is to define, in molecular terms, the nature of the metabolic block in virulent Yersinia pestis, Yersinia pseudotuberculosis, and Yersinia enterocolitica which results in in vitro restriction of growth at 37 C but not at 26 C unless the medium contains physiological levels of Ca++ (2.5 mM). Emphasis will be placed on defining the role of the plasmid-mediated V and W or virulence antigens which are only induced in restricted cells and which probably account for that restriction by promoting shutoff of stable ribonucleic acid synthesis and reduction of adenylate energy charge. The role of procaryote Ca++ binding proteins, calcium-dependent and independent phosphodiesterases, and calmodulin-like activities in promoting restriction and in favoring penetration and growth within mammalian cells will be examined. The rationale for the ability of exogenous nucleotides to promote Ca++ independent multiplication will be defined as will any role of the virulence antigens in mediating this effect. The physical and chemical properties of V antigen (known to cause immunity to all yersiniae) and W antigen (a unique procaryote lipoprotein) will be described in detail. Formal proof of the protective effect of monospecific anti-V will be obtained by demonstrating its ability to promote passive immunity. The capacity of yersiniae to penetrate and grow within professional and nonprofessional phagocytes will be compared in the presence and absence of anti-V serum and the influence of such protective antibody on dissemnation of the organisms in vivo will be determined. The possibility that lipoidal W antigen inhibits intracellular killing by established mechanisms will be determined as will the putative invasive roles of ancillary outer membrane peptides unique to virulent Y. pseudotuberculosis and Y. enterocolitica. Resolution of these questions will provide a broad foundation for understanding the molecular basis of virulence in general facultative intracellular parasites.