Effects of in ovo injection of L-carnitine on the hatchability, grow-out performance, and slaughter yield of Ross x Ross 308 broilers from a young breeder flock were determined through 48 d of age. Fertilized eggs were injected in the amnion with L-carnitine (0.5, 2.0, or 8.0 mg dissolved in 100 microL of a commercial diluent) on d 18 of incubation using an automated egg injector. Three control groups (noninjected and injected with or without diluent) were also included. Hatchability and hatch rate of fertilized eggs were assessed. Furthermore, subsequent mortality, BW gain, feed intake per bird, and feed conversion were determined through 46 d posthatch. On d 47, live body, carcass, and abdominal fat pad weights, along with the weights of all major commercial cuts including the thigh, drumstick, wings, and breast muscles, were determined. Individual doses of supplemental L-carnitine had no significant effect on the hatchability or rate of hatch of fertilized eggs; however, significant trends were noted for increased hatchability and length of egg incubation in conjunction with increases in L-carnitine dose. Nevertheless, there were no significant treatment effects on any of the grow-out performance or slaughter yield parameters investigated. In conclusion, although increasing the levels of L-carnitine added to commercial vaccine diluent between 0.5 and 8.0 mg/100 microL for commercial in ovo injection did not significantly influence subsequent broiler grow-out performance or slaughter yield, L-carnitine dosages above those used in this study have the potential for significantly increasing incubation length and hatchability of broiler hatching eggs.