Effect of a 510.6 nm laser on the proliferation of hematopoietic cells in human umbilical cord blood was studied. Mononuclear hematopoietic cells were prepared from human umbilical cord blood, and cultured with CSF in vitro, the GM-CFUc was 18.6±12.6/104. If the mononuclear cells were cultured with CSF which was prepared by laser irradiated cells, the GM-CFUc was 40.5±20.2/104. If the mononuclear cells were irradiated by laser and then cultured with CSF, the GM-CFUc was 36.5±18.5/104. If both the mononuclear cells and the cells for CSF preparation were irradiated by laser, the GM-CFUc was increased significantly, it was 57.4±41.2/104.