Abstract Chronic administration of indomethacin to weanling (20 day old) BALB/c mice inoculated with Moloney sarcoma virus (MSV) delayed the onset of the tumor and suppressed the tumor growth. As expected, the prostaglandin (PG) levels associated with the MSV-injected leg muscle of the indomethacin treated mice were greatly depressed when compared to the elevated PG content associated with the tumors of MSV injected control mice. There was no effect of indomethacin on the cyclic AMP or cyclic GMP levels. The elevated levels of PG and cyclic AMP associated with the tumor were found to parallel the tumor growth. Administration of antilymphocyte serum (ALS) to the mice at the time of MSV inoculation resulted in accelerated and enhanced tumor growth. Indomethacin treatment of these mice similarly suppressed the tumor growth, but less dramatically.