The injection (s.c. or i.p.) of 10(6) live or lethally irradiated methylcholanthrene-induced fibrosarcoma cells into CBA/Ca mice one or 2 days before i.v. challenge with the same tumour inhibited the formation of artificial lung tumour metastases. In addition, it also frequently enhanced the cytostatic effect of peritoneal exudate cells on monolayers of the same tumour. The effects on lung tumour metastasis were not noted if X-irradiated tumour was injected i.v., or if s.c. administration was delayed until one day after i.v. challenge. Similar effects on tumour growth were also observed in C3Hf/Bu mice and (CBA/Ca x A/HeJ) F1 hybrids which were pretreated (s.c.) with tumour shortly before i.v. challenge with the same tumour. Further studies in CBA/Ca mice suggested that the protective effect was tumour-specific, for the growth of i.v. injected tumour was not significantly inhibited by pretreatement with a number of other MC-induced or spontaneous tumours from the same and different strains
