SN11841 [4'-(9-acridinylamino)-methanesulfon-m-aniside] is an antitumor compound discovered by B.F. Cain. The LD50 for BALB/c mice with single intraperitoneal dosage is approximately 25 mg/kg. RLV-(Rauscher leukemia virus)-induced splenomegaly, a disease indicator in BALB/c mice, is inhibited at SN11841 doses not causing acute mortality. The life span of RLV-infected mice increases at some SN11841 doses. SN11841 does not have direct, or virolytic effects on RLV under conditions approximating those of antiviral effectiveness. SN11841 is cytotoxic for cells in tissue culture, as measured by inhibition of growth rate or vital dye uptake. At nontoxic concentrations SN11841 has no effect on RLV infectivity for murine cells, as determined by XC-cell induced syncytium formation. SN11841 has antiviral activity against vaccinia virus in tissue culture but is inactive against herpes simplex (Type 1), vesicular stomatitis, encephalomyocarditis, or reoviruses. SN11841 apparently does not act by inducing interferon. SN11841 is chemically labile, particularly in the presence of sulfhydryl compounds, but the degradation products resulting from prolonged storage in media are neither cytotoxic nor antiviral.