An experiment is described which measures the collision cross section for transfer of excitation in rubidium by helium. Rubidium atoms are pumped to either the 2P12 or 2P32 state by irradiation with one of the resonance lines. Some excited rubidium atoms are transferred to the other P state by collisions with helium atoms. The rubidium atoms then radiate both resonance lines. Measurements of the intensities of both resonance lines as a function of helium pressure are used to calculate the collision cross section. The cross section for 2P12 to 2P32 is 1.0×10-17 cm2+/-10% and for 2P32 to 2P12 is 1.2×10-17 cm2+/-10%.