In this essay, I propose a new method of treating tumours, using an old and inexpensive preparation, that I contend would be of considerable benefit to patients and their cancer management. My rationale for this treatment initially arose from recent advances in the understanding of dendritic cell function. (Dendritic cells are key cells of the immune system that are able to either turn on or turn off T-cell responses.) Evidence to support this approach is found in 100-year-old studies on the immunotherapy of cancer. Also, I draw on some remarkable, but little-known studies from the 1960s-1990s, demonstrating that the preparation has already been trialled in humans (although not intratumourally, as I propose), and is considered sufficiently safe to proceed with clinical trials in cancer volunteers.