A cell-free system from Xenopus eggs mimics the reaction occurring at the eukaryotic replicative fork in vivo when chromatin assembly is coupled to complementary strand synthesis of DNA. DNA synthesis on single-stranded circular DNA promotes supercoiling and the replicated molecule sediments as a discrete nucleoprotein complex. Micrococcal nuclease digestion reveals a characteristic pattern of multiples of 200 bp of DNA. The kinetics of chromatin assembly and DNA synthesis are coincident and both processes occur with a rate comparable with chromosomal replication in vivo in early embryos. The DNA synthesis reaction can be uncoupled from the assembly reaction. Thus, titration of chromatin proteins by preincubation of the extract with double-stranded DNA prevents the supercoiling of replicated DNA without affecting the rate of synthesis. In contrast, chromatin assembly performed on unreplicated double-stranded DNA is a slower process as compared with the assembly coupled to DNA synthesis. Supercoiled molecules are detected after 30 min replication whereas at least 2 h are required to observe the first form I DNA with unreplicated double-stranded DNA. Such a system where chromatin assembly is promoted by DNA synthesis should be valuable for studying the interaction of specific factors with DNA during chromatin assembly at the replicative fork.