Mice bearing the B16 melanoma or treated with Corynebacterium parvum develop elevated levels of plasma neutral proteinase activity. Similar experiments carried out with C57BL/6-bg/bg (beige) mice, which are genetically deficient in polymorphonuclear neutrophil (PMN) proteinases, revealed that such mice develop significantly diminished elevation in plasma proteinase activity compared to C57BL/6-bg/+ mice. Lysates of C. parvum elicited PMN from beige mice contained approximately 80% less neutral proteinase activity as did lysates of PMN from bg/+ mice. These results indicate that host cells, such as PMN, may become activated during the tumor progression, or following C. parvum treatment, causing degranulation and a subsequent elevation in plasma proteinase levels. If such an interpretation is correct, then this phenomenon may be the murine corollary to what has been observed in patients with certain inflammatory diseases or tumors.