The present investigation was under taken to study whether the tumor-associated macrophages(TAM) of Daltons lymphoma (DL), a spontaneous transplantable murine T cell lymphoma can beactivated to tumoricidal state by crude thymus extract. Intraperitoneal administration of thymusextract to DL-bearing mice resulted in activation of TAM with an enhanced IL-1, TNF and antigenpresenting ability. It was found that treatment with thymus extract could also enhance thephagocytic and cytotoxic activity of TAM. However, only a marginal increase in arginase activitywas observed. Till date to the best of our knowledge the effect of crude thymus extract on theactivation of tumor associated macrophages has not been investigated, this study provides a newpiece of information in the area of thymus therapy.