Sulphur mustard (SM), commonly known as mustard gas is an alkylating agent that causes serious blisters upon contact with human skin. SM is frequently used as a chemical warfare agent. There is some evidence for sulfur mustard-induced lymph system effects in humans. Between 2000-2001, 42 male albino Wistar rats were used. After accommodation with environment, we divided rats to control, sham and experimental groups (2.5 mg/kg, 5 mg/kg, 10 mg/kg, 20 mg/kg and 40 mg/kg). Then we injected sulphur mustard oil in rat's intraperitoneal space. Then their spleens were removed for histological verification. Our results showed that significant difference in lymphocytes number in experimental groups after 24 hours. The number of lymphocytes in 5, 10, 20 and 40 mg/kg groups was increased and this increase in 40 mg/kg group was more than the other groups. We concluded that the number of lymphocytes increased due to exposure of mustard gas and there is a relationship between the increase of lymphocytes and dose of exposure.