[en] The molecular weight, shape, composition, subunit structure, crystal form and electron microscopy of a mitogenic erythroagglutinating phytohemagglutinin from Phaseolus vulgaris were determined. It was established that its mitogenic activity is due to the protein itself and not due to a non-protein contaminant. The relationship of the structure to function of this phytohemagglutinin has been investigated. It was established that not all subunits are mitogenic and that amino groups are indirectly involved in the mitogenic site through stabilization of its conformation. The mechanism of its mitogenic activity was studied. It was found that it does not act as an ubiquitous antigen universally sensitizing lymphocytes. It is first bound to the cell membrane and subsequently is internalized and localized in the heterochromatic areas of the nucleus. The biochemical basis of the radiosensitivity of human blood lymphocytes and of the radioresistance of human polymorphonuclear gramulocytes was investigated. Data so far obtained indicate that the ability of the cells to reduce ascorbate is a significant contributing determinant of radio-resistance, and that the increased radioresistance of transformed lymphocytes is due to their higher content of enzymes inactivating oxidants and free radicals