Golden hamster eggs fused with human sperm were pulsed with bromodeoxyuridine to determine the timing of S‐phase and the length of the first ceil cycie in this hybrid cross. Fused eggs were fixed and pronuclei scored for incorporation of the thymidine analogue detected by indirect immunofiuorescence. Although S‐phase started synchronously 3–3.5 hr after coincuba‐tion of sperm and eggs, its duration was variable such that two‐cell stages appeared at 16 hr while a proportion of pronuclei was still engaged in DNA synthesis. Unlike rodent sperm chromatin, human sperrn chromatin was able to participate in DNA synthesis well before its maturation into a fully developed pronucleus. Human sperm chromatin appears able to function under conditions different in several respects from those in human eggs.