27Extracellular mRNAs (ex-mRNAs) potentially supersede extracellular miRNAs (ex-miRNAs) and other 28 RNA classes as biomarkers. Here, we present a comprehensive extracellular RNA (exRNA) study in 29 human blood circulation based on conventional small RNA-sequencing (sRNA-seq) and sRNA-seq after 30 T4 polynucleotide kinase (PNK) end-treatment of total exRNA isolated from serum and platelet-poor 31 EDTA, ACD, and heparin plasma. Applying strict criteria for read mapping and annotation, we found that 32 compared to conventional sRNA-seq PNK-treatment increased the detection of informative ex-mRNAs 33 reads up to 50-fold. Based on captured ex-mRNAs from healthy individuals, we concluded that the 34 exRNA pool is dominated by hematopoietic cells and platelets, with additional contribution from the 35 liver.