Distributions of AT- and GC-base pairs along the length of chromosomes 1, 2, 3 and 16 in primary cultures of embryonic fibroblasts and of peripheral blood leucocytes were studied by autoradiography with: 1. 3H-thymidine and 3H-deoxycytidine; 2. 3H-deoxyadenosine and 3H-deoxyguanosine. It has been shown that the two types of cells differ in the DNA content and proportion of AT- and GC-nucleotide pairs in the centromeric heterochromatin of chromosome I: this region contains more DNA in fibroblasts than in leucocytes mainly due to AT-pairs. In both types of cells the telomeric region of the short arm of this chromosome contains more GC- than AT-pairs. Similar results were obtained for C-heterochromatin of chromosome 16: the frequencies of labelling of this region by 3H-deoxyadenosine and 3H-thymidine in fibroblast cultures were higher than in case of 3H-deoxycytidine and 3H-deoxygyanosine, and in leucocyte cultures these frequencies were almost equal. No differences in the distributions of base pairs along the length of chromosome 2 and 3 were established in the two types of cells. -- The nature of the established phenomenon may be connected with under-replication or loss in another way of part of the genetic material in the process of development and differentiation of cell systems.