Extensive evidence supports a role for polyclonal serum natural antibody (NAb) acting as a mediator of natural resistance against tumors in mice. However, little is known about its mechanisms of action or about the phenotype of susceptible cells. C3H 10T${1\over 2}$ fibroblasts overexpressing an activated ras oncogene or a PKC-$\beta$1 gene increased their NAb binding capacities, identifying PKC, an integral signaling molecule of normal cellular activation, as a key regulator of NAb binding structures. This, coupled with corresponding decreases in expression of membrane PKC-$\alpha$ and NAb binding in resting confluent 10T${1\over 2}$ cells raised the possibility that, in general, cells activated through PKC are NAb sensitive. In addition, NAb interaction with 10T${1\over 2}$ variants initiated a signal transduction mechanism including activation of PKC, shedding of cell surface molecules and bound NAb, a reduction in phosphotyrosine levels of a membrane-associated 60 KDa molecule, and, over time, the inhibition of DNA synthesis. Together with the increased in vivo elimination of the high NAb binding PKC-$\beta$1-overexpressing cells and the beneficial effect of passive syngeneic NAb in the rejection of syngeneic tumors injected s.c. and i.v. in both xid-bearing B cell deficient and B cell normal mouse models, the data argued that NAb not only participates in tumor surveillance of preneoplasia and neoplasia but contributes to homeostasis of the organism.