The effect of Rauscher Leukemia Virus (MuLV-R) infection on the lipid composition of mouse spleen from BALB/c mice was investigated. Drastic changes in the lipid composition of the spleen as a result of tumor growth induced by the virus could be demonstrated at 21 days after infection. The molar ratio of cholesterol to phospholipids was found to be low, while a shift within the choline containing phospholipid classes resulted into a lower sphingomyelin and a higher phosphatidyl choline content of the MuLV-R infected spleen. The cholesterol ester content increased more than two-fold during tumor growth, and shifts in the fatty acid patterns of the lipids were demonstrated.