The erythropoietin activity and the erythropoiesis inhibiting factor(s) (EIF) were studied in the serum of 10 children and adolescents suffering from terminal renal insufficiency. The influence of haemodialysis on these factors was examined as well. As a test model for the estimation of erythropoietin activity and EIF we used hypertransfused polycythaemic mice. No erythropoietin activity was detectable in the serum of children and adolescents. A 40%-inhibitory effect on erythropoiesis existed in 8 out of 10 uraemic sera prior to dialysis. After haemodialysis the inhibition is eliminated in 4 of the 9 sera examined, and reduced to half in the 5 remaining sera. The findings suggest that the inhibitory effect is possibly caused by one or several EIF of the middle molecule group.

The accessibility of the two complementary DNA strands in newly replicated chromatin of Ehrlich ascites tumor (EAT) cells grown under conditions of cycloheximide-inhibited protein synthesis was studied by analysis of the DNase I digestion of isolated nuclei. Bulk DNA was labeled with 14C-thymidine and the newly synthesized strands - with bromodeoxyuridine and 3H-thymidine. The DNase I digests were fractionated in two successive CsCl density gradient centrifugations to obtain a dense fraction containing 15-20% newly replicated DNA. Analysis of the distribution of 14C-labeled parental DNA fragments complementary to the 3H-nascent strand has shown that the 14C-labeled fragments prevail in the region of 30-50 nucleotides. Simulation experiments using the rate constants for DNase I attack show that this result may be explained by an enhanced accessibility at the nucleosomal 5'-end region of the parental strands, where the H2a-H2b dimer interacts with DNA. This asymmetry seems to be induced by interactions in the chromatin.