In 1939, when the Bureau of Dairy Industry began its crossbreeding experiment at Beltsville, the published information on the crossbreeding of dairy cattle was slight, unimpressive, and very little of it was based on experimental results. The crossbreeding investigations at Beltsville were undertaken to develop knowledge on the subject which would provide a sound basis for advising the operators of milk-producing farms what to expect if they used crossbreeding in hopes of producing better replacements for their milking herds. Final results on any dairy cattle breeding research project cannot be published until the last animal on the project has died, so this report covers the breeding performance and milk and butterfat production on all individuals used on the project which have had time to complete a normal first-lactation period.