In order to investigate at ultrastructural level the mechanism of DNA synthesis progression during the different moments of S-phase a bromodeoxyuridine-anti bromodeoxyuridine (BrdU-anti BrdU) method has been applied to synchronized 3T3 fibroblasts. After 30 min BrdU incorporation, five different labelling patterns can be identified and should be related to early, middle and late S-phase. These patterns are represented mainly by diffuse labelling localized in different nuclear domains and by quite rare cases in which the labelling is limited to isolated clusters of gold particles. After a 5-min pulse with BrdU it is possible to observe isolated clusters of gold particles at each moment of S-phase, which, however, exhibit the same distribution of the five principal labelling patterns observed after 30 min incorporation. In both cases labelling can be detected in the interchromatin regions during early S-phase, at the boundary between interchromatin and heterochromatin during middle S-phase and in the heterochromatin domains during late S-phase. Considering their size, the isolated spots of labelling could be interpreted as single replication units which are subsequently activated throughout the different moments of the S-phase.