The morphological aspect of the antineoplastic defence mechanism was studied in Sewall-Wright strain 13 guinea pigs using isogeneic line 2 hepatoma cells. When these tumour cells were injected i.p. into isogeneic hosts, they grew rapidly and always killed their hosts within 8 weeks. An intradermal injection of line 2 hepatoma cells in strain 13 guinea-pigs 3 weeks before i.p. challenge with the same tumour cells protected such pre-immunized animals. While the intradermal inoculum failed to grow, it conferred sufficient immunity to enable the host to reject the tumour after a short period of growth. This phenomenon showed immunological specificity since intradermal injection of allogeneic hepatoma cells did not offer this protection. By using sequential histological analysis in these preimmunized animals, we were able to identify 2 sets of cells migrating into and around tumour modules in relation to tumour cell death. Firstly, small numbers of small lymphocytes were observed at the earliest signs of tumour cell damage. They were most likely the effectors of antineoplastic defence mechanisms. Secondly, a variety of numerous inflammatory cells including macrophages were observed later when the tumour was necrotic, and these cells were considered to be present as a response to tumour cell death.