Background:, a fruit used by poultry farmers in some parts of Nigeria to combat some poultry diseases such Cucumis metuliferus as Fowl Typhoid, Gumboro and Newcastle disease. The people of Plateau State in Nigeria eat the fruit believing that it cures HIV/AIDS. White blood cells are known to fight against diseases, hence the need to study the effect of this fruit on white blood cells. To find out the effect of the prolonged administration of the extract on total white blood cell (WBC) Hypothesis/Purpose: count, differential leucocyte count (DLC) and absolute differential leucocyte count (ADLC) was calculated. This will establish the plant use in the treatment of diseases. Twenty seven weeks old cockerels were used for this study. They Material and methods: were randomly divided into four groups of 5 chicken each (groups A, B, C and D). Cockerels in group D served as the untreated control and were given only feed and distilled water daily for a period of 28 days. The cockerels in groups A, B and C were treated daily orally with graded doses of crude methanol extract (CME),(200, 400 and 600 mg/kg respectively). There was a Results: significant (p< 0.05) increase in the value of total WBC from day 14 in all the treated groups while those that were given 600 mg/kg had a significant (p< 0.05) increase from day 7. The result of the DLC showed that there was a significant (p< 0.05) increase on lymphocytes percent in all the treated groups from day 7 when compared to their day 0, while the percentage of monocytes, neutrophils, eosinophils and basophils showed a significantly (p< 0.05) decrease in all the treated groups. The result of absolute DLC showed a significant (p< 0.05) increase in the values of lymphocytes, monocytes, neutrophils and eosinophils, while that of basophils showed no significant difference throughout the periods of experiment in all the treated groups. The results Conclusion: of this study showed that the plant increased the level of white blood cells and may probably stimulate the Cucumis metuliferus immune system.