An objective is to provide screening methods for substances having an activity of promoting maturation into hemocytes. It was revealed that in the maturation process of erythrocytes and platelets, RP S19 multimers are released to the outside of the cell and act on the C5a receptors of erythroblasts and megakaryocytes, thereby causing maturation of erythroblasts and megakaryocytes to erythrocytes and platelets. Based on such findings, the present invention provides methods of screening for substances having an activity of promoting maturation into hemocytes by targeting the RP S19 multimers and C5a receptors. The screening methods of the present invention can detect hemocyte maturation using an increase of the R4 RGS family as an indicator. Furthermore, the present invention provides agents for promoting hemocyte maturation, which comprise RP S19 multimers and a substance having an activity of promoting maturation into hemocytes as useful components. The present invention also provides methods for producing erythrocytes and platelets, which include the step of contacting RP S19 multimers and a substance having an activity of promoting maturation into hemocytes with erythroblasts or megakaryocytes.