Labelled RNA extracted from human peripheral lymphocytes was digested with DNase and RNase and chromatographed on cellulose columns. The results show that a significant proportion of the RNA synthesized in both unstimulated and PHA-stimulated human lymphocytes is in an RNase-resistant form with the properties of double-stranded RNA (ds-RNA). The proportion of total labelled cell RNA which appears in double-stranded form in unstimulated lymphocytes is much greater than in PHA-stimulated cells, presumably due to the much larger synthesis of ribosomal RNA in the latter cells.