A gel chromatographically uniform extract of soluble peptides of thymus (thymus fraction A) of molecular weight about 10,000 is separated into further six subfractions with a HPLC system developed in our laboratory for preparation of peptides. Thymus fraction A induces a modjrate proliferation of human lymphocytes as assessed by the incorporation of H thymidine into the DNA yielding a maximal stimulation index of 15.4 at the 4th day at a concentration of 50 ug/ml. Except factor I which causes a slight but significant lymphocyte prolifera­tion,the factors II to VI do not show any stimulating activity. Therefore, besides the well known differentiation-inducing capacity, thymus fraction A has also a cell division-inducing activity.