Provided is a treatment method for damaging an erythrocyte and a leukocyte while suppressing damage to cells other than blood cells present in blood. In an embodiment, the disclosure relates to a method for treating a sample containing blood components, the method including mixing a sample containing blood components with a surfactant A, where the surfactant A is a nonionic surfactant represented by General formula R1—O-(EO)n-R2 (I).