[header] How to diagnose and treat tumors in guinea pigs [title] Locate any lumps on its body. [step] Take your guinea pig out of its cage and look over its body for any raised, rounded lumps. Use your fingers to brush through its fur to feel for any abnormal growths on its skin. 
[substeps] Lumps that appear on the belly of your guinea pig may be mammary tumors. They can appear on male or female guinea pigs.