Diplochromosomes from colchicine-induced endoreduplicated HeLa cells were analyzed by light and electron microscopy. The patterns of C-, G-, NOR-, and late replicating DNA-rich bands obtained clearly indicate that the four chromatids constituting a diplochromosome are structurally identical and behave the same, at least in terms of replication kinetics and r-DNA transcription. Under the electron microscope, whole-mounted diplochromosomes appeared formed by two completely divided chromosomes showing no physical connection at their centromeric regions. The two chromosomes forming a diplochromosome seem to be held together by chromatin fiber loops connecting the two neighbouring chromatids of the sister chromosomes. These connecting loops clearly decrease in number as chromosome condensation progresses.