A new double-label histochemical method is described which permits accurate quantitation of macrophage recruitment into neoplastic lesions in situ. Intratumoral macrophages are identified by their capacity to ingest colloidal iron particles from the interstitial fluid. Since colloidal iron is retained in a stable form within these cells for a considerable time, new macrophages that emigrate into the tissue after injection of the colloidal iron are identified by their ability to ingest a second colloid (lanthanum) which can be reliably distinguished from the initial iron label. Preexisting (colloidal iron label) and newly recruited macrophages (lanthanum label) are identified in serial sections by histochemical methods using hydrogen peroxide oxidation to detect iron (blue reaction product) and cleavage of phosphate esters to demonstrate lanthanum (magenta reaction product). The macrophage content and macrophage recruitment were found to vary substantially in individual metastases within the same host.

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