Lymphokine-rich supernatants from concanavalin A-stimulated spleen cell cultures failed to activate mouse peritoneal macrophages for tumor cell killing, providing that the assay system was free of detectable (less than 0.125 ng/ml) endotoxin. Instead, increasing amounts of lymphokine progressively increased the sensitivity of macrophages to activation by purified bacterial lipopolysaccharide (LPS). Conversely, the effect of minute amounts of lymphokine could be detected in the presence of relatively high (though nonactivating) concentrations of LPS. The latter observation was exploited to develop a highly sensitive, quantitative assay for lymphokine. In this assay, serially diluted supernatants were tested on macrophage monolayers in the presence of a constant concentration (3 ng/ml) of LPS. Under these conditions, the lymphokine dose-response curve was sigmoidal and therefore suitable for conversion to a linear plot using the logarithmic transformation of the von Krogh equation. Activity could be expressed quantitatively and reproducibly in terms of units of activity. By facilitating purification the assay should contribute to the development of a more precise biochemical understanding of the role of lymphokine in macrophage activation for tumor cell killing.

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