A lump under the skin of the biceps can have many different causes, though some of the more common causes are cysts, dermatofibromas and lipomas, reports WedMD. Understanding the characteristics of each helps determine the cause of a lump.
Cysts are smaller, usually painless and usually "roll" under the skin of the biceps when touched. Dermatofibromas are darker in color, hard to the touch and may cause pain and itching. Lipomas are slow-growing, benign tumors that are notably soft to the touch, especially on the biceps muscle, according to WebMD. All three types of lumps can be removed for cosmetic reasons, or if they become painful. An individual should seek medical attention for any lump that shows signs of sudden growth, discoloration or has uneven edges.