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Previous work from this laboratory has shown that preimmunization of syngeneic hosts with Rous sarcoma virus (RSV)-transformed cells elicits a strong immune response against the growth of transplantable RSV sarcomas, mediated by T lymphocytes expressing the surface phenotype of helper cell precursors (Prat, Di Renzo & Comoglio, 1983). This paper shows that anti-tumour immunity may be elicited in tumour-bearing animals by triggering an experimentally pre-amplified T-helper cell population at the site of tumour growth. Mice were treated with cyclophosphamide (which inactivates suppressor T cells) followed by skin sensitization to trinitrochlorobenzene (TNCB) according to a protocol that has been shown to induce an appreciably amplified generation of trinitrophenyl (TNP)-reactive helper T cells (Fujiwara et al., 1984). Five weeks after TNCB painting, mice were transplanted s.c. with a lethal dose of RSV-induced syngeneic sarcoma cells; the injection at the tumour site of TNCB induced the regression of the tumour in mice in which the TNP-helper cell population has been amplified, but not in controls, including those injected with a non-related hapten or sensitized to TNCB without inactivation of suppressors. [ABSTRACT FROM AUTHOR]