A comparison of the sequence of DNA replication in an isodicentric (idic) X chromosome was made between peripheral blood lymphocytes and skin fibroblasts from a 33-year-old female with primary amenorrhea, somatic stigmata of Turner syndrome, and normal stature and intelligence. The patient had a karyotype 45,X/46,X,idic(X)(q27.1) to lymphocytes and 46,X,idic(X)(q27.1) in skin fibroblasts. Both centromeric regions of the idic X showed C-staining but only one primary constriction. BrdU-33258 Hoechst-Giemsa techniques were used to analyze regional DNA replication patterns. The idic X chromosome was always late replicating in lymphocytes and skin fibroblasts, except that about 1-2% of cells completed replication simultaneously in both normal and idic X chromosomes. Fifty-six percent of the asymmetric patterns in lymphocytes showed an equal proportion of early and late functional and non-functional centromere halves. In skin fibroblasts, 60.8% of cells were asymmetric: the functional half tended to replicate later than the non-functional half. Some differences were observed between these two cell types. As examples, band q23 was late replicating in lymphocytes, but early replicating in fibroblasts; q25 was intermediate to late replicating in lymphocytes, but one of the last bands to complete replication in fibroblasts. Thus, different cell typed influenced the replication kinetics of the idic(X). Furthermore, several variants of the replication sequence were found in both cell types. The findings support the hypothesis that the control of DNA replication in the inactive X chromosome is multifocal, and suggest that the active idic X chromosome replication may reflect a relative lack of self-control or heterogeneity of cell population.