Determination of radioactivity has been made on 43 rocks, carefully selected from the different mapped units of the complex Cretaceous batholith of Southern California; they range from gabbro to granite. The activity of the gabbro averages about 0.3 alphas/mg./hr., that of the tonalites, 0.8, the granodiorite 1.3, and the granites over 2.0. The activity of the average rock of the batholith is about 0.9 alphas/mg./hr. Most of the rocks have been analyzed, and modal compositions are given. The radioactivity and the percentages of K2O, SiO2 and PbO are plotted on a variation diagram. The variation curve for activity nearly parallels that for K2O, and the ratio of activity to K2O is about 0.5. The radioactivities of some rocks fall far from the variation curve, and degree of activity correlates with K2O content. A less-marked correlation between activity SiO2 and PbO was also noted. The K2O, SiO2, PbO, and radioactivity concentrate in the residual liquid during crystallization and differentiation. PbO follows K2O closely, as it is concentrated in the K2O minerals, but radioactivity is very low in the K2O minerals and is greatly concentrated in the accessory minerals.