A technique has been developed for injecting known amounts of liquids into fertilized one-celled mouse eggs by use of a calibrated ocular micrometer superimposed on the terminal regions of the injection pipette. From 128 pronuclear eggs, each injected with 180 or 770 cubic microns of bovine gamma globulin at a concentration of 25 milligrams per milliliter in citrate-Locke's solution and then transferred to the oviducts of pregnant foster mothers, 18 living fetuses developed; from 74 eggs, each injected with 2730 cubic microns, 5 fetuses survived. The living fetuses that developed from the injected eggs were smaller than normal in 6 of 23 surviving experimental fetuses as contrasted to only 1 of 19 control fetuses.