Long-term infection of mice with Trichinella spiralis has been shown to stimuate increased host protection aginst transplantable solid tumors. The present investionation was initiated to determine whether parasitized mice were similarly protected from induction and progression of an ascites tumor. ICR/CD-1 mice were orally infected with 50, 100, 200, 300, or 400 T. spiralis larvae, and subsequently challenged intraperitoneally with 5 X 10(4) Sarcoma-180 (S-180) ascites cells. Animals were observed and weighed daily for development and progression of malignancy. Protection from fatal ascites neoplasia was found to be statistically significant under selected conditions of larval dose and challenge interval. Mice infected with 100 or 200 larvae were more resistant to S-180 progression than both uninfected controls or other infected groups. Protection was observed in groups challenged 2 weeks after nematode intubation but not at 6, 8 or 34 weeks. This finding is in contrast with long-term protection seen previously with B-16 melanoma tumors, and suggests that antineoplastic effects of T. spiralis infection differ for ascitic and solid murine tumors.