The isotopic composition of lead in feldspar varies widely from pluton to pluton of the Late Cretaceous Boulder batholith, encompassing the following ranges in isotopic values: 16.9-18.1 for Pb ²⁰⁶/Pb ²⁰⁴; 15.4-15.7 for Pb ²⁰⁷/Pb ²⁰⁴; and 37.7-38.5 for Pb ²⁰⁵/Pb ²⁰⁴. Although each pluton has a characteristic isotopic composition, the fact that Pb ²⁰⁶/Pb ²⁰⁴ for the Butte Quartz Monzonite varies by 1.3 percent, compared to a range of about 8.6 percent for the batholith as a whole, suggests that perfect isotopic mixing of the magma was not always attained. Whole rock initial Sr ⁸⁷/Sr ⁸⁸ for the entire batholith ranges from 0.705 to 0.710, comparable to ranges for the Sierra Nevada and British Columbia batholiths. Ore leads of the Butte district are isotopically very similar to feldspar leads of the host Butte Quartz Monzonite. This isotopic similarity was interpreted by Murthy and Patterson (1961a) to indicate complete mixing of independently derived feldspar lead and ore lead rather than close genetic relationship. However, detailed study of the only sample in the present investigation, from the Donald pluton, that clearly has mixed leads shows significant differences in lead isotopic composition between megacryst K-feldspar and both the groundmass K-feldspar and plagioclase, indicating that isotopic mixing of leads is not complete, even within a single hand specimen. Spatial relations of isotopic differences among the various plutons strongly suggest that complete mixing of leads of different isotopic composition from the magma did not occur in the Butte Quartz Monzonite nor in any other pluton of the batholith, and that the isotopic similarity between Butte ore and feldspar lead of the host rock indeed stems from a genetic association. Lead and strontium isotope ratios generally behave in a similar way; that is, the more radiogenic the lead in a rock, the more radiogenic the strontium. The source of the lead which could produce the isotope variations observed for the batholith is calculated to have a mean age of about 2,200 m.y., which is compatible with ages of Precambrian crystalline rocks in the region, and an isotopic makeup with U ²³⁸/pb ²⁰⁴ <9, Th/U > 4, and Rb/Sr of 0.03-0.09. However, no large volumes of prebatholith rocks exposed in the Boulder batholith region are of the required isotopic composition, which is typical of basaltic-gabbroic or quartz dioritic chemical composition. Models involving complete melting of possible source materials to account for the isotope variations are considered and rejected. Mechanisms involving par.tial melting of lower crustal or upper mantle source material appear to be the most viable in explaining the observed isotopic compositions. Assimilation of upper crustal material (i.e., the Precambrian Belt and pre-Belt rocks, in which the lead and strontium are more radiogenic than in the batholith rocks) may have accompanied partial melting of the lower crust or upper mantle and indeed must have played a major role if the source material had a composition isotopically comparable to that observed for oceanic tholelites (i.e., low Th/U, U ²³⁸/Pb ²⁰⁴ and Rb/Sr), but would be relatively unimportant if the source material were sufficiently radiogenic to begin with as might be expected from basalt, gabbro, or quartz diorite compositions 2,200 m.y. old. © 1968 Society of Economic Geologists, Inc.