Current methods of physical mapping allow the estimation of genomic distances (i.e. DNA contents) from linear distances between DNA markers in interphase nuclei, and in this study we estimated the size of focal centers of DNA replication in cultured S-phase human cells. Our results indicate that the conformation of S-phase chromosome fibres in the range of contour lengths 0.1-3.0 microns fits the random walk model and, therefore, the quantitative methods of interphase mapping can be applied to the estimation of sizes of replication units. The obtained data show the existence of multiple non-clustered small units less than 150 kb in size, equivalent to small replicons detected by fiber DNA radioautography, and also a significant fraction of big units more than 500 kb in size, representing groups of small replicons and/or big replicons. These big units are detected as chains of small replication foci, probably reflecting the structural chromatin organization in well-known loop domains, since the experimentally induced decrease of replicons to the average size 12 kb does not lead to any change in the pattern of indicated chains.