Whole rock trace element analyses of conglomerate clasts from the _kc;lfu Cretaceous Cabrillo Formation were determined for 25 volcanic clasts and 6 granitoid clasts to investigate possible source areas for the clasts. Strontium, zirconium, rubidium, titanium and silica contents were analyzed by X-ray fluorescence done at San Diego State University and at Otago University in New Zealand. The average Sr contents for volcanics and granitoids were 113 and 180 ppm respectively. These concentrations overlap with those reported from the Santiago Peak Volcanics (SPV) and western zone plutonic rocks of the Peninular Ranges batholith (PRB) and are much lower than typical values from eastern zone La Pesta-type plutonic rocks of the PRB. Rubidium contents however are much higher than western zone plutons. Titanium concentrations are consistent when compared to SPV, but zirconium contents may distinguish the conglomerate clasts from SPV. This study is unable to confirm a close compositional similarity between Cabrillo Formation conglomerate clasts and PRB plutonic and volcanics suites exposed at the present erosional level.