Clarias lazera fish were used to study the toxicity of the triad combination of lead, mercury and arsenic at 1/100 of their LC50/72 h. for 5 wekks. All fish exposed showed decreased and unbalanced movements, appearance of red patches on the skin accompanied by inflammation laceration. Some fish showed presence of thick mucus film over the and gills. Darkening of the skin colour was also observed. There were significant decrease in red blood cell counts from the second week of the experiment till the end, accompanied by decreased haemoglobin concentration. These resulted in macrocytic hypochromic anaemia. These results could be explained as a suppressive effect of haevy metals on both haem and globin formation. The white blood cell counts showed significant increase in their values from the second week of exposure till the end of the experiment. Significant decreased values of serum total protein and total cholesterol were observed from the second week of exposure. The observed results could be explained according to the toxic role of the haevy metals on the hepatic parenchyma and cholesterol synthesis, estrification and excretion from the liver. There were significant increased values of serum glucose in the first three weeks, followed by significant decrease in the last two weeks. The hyperglycaemia observed may be due to impairment of carbohydrate metabolism, and the latter hypoglycaemia could be explained as a result of hepatocellular destruction due to exposure to heavy metals. Activities of serum transaminases showed significant increased values during the experiment. Liver showed the highest significant residual contents for the three heavy metals, followed by the kidney. The muscle tissue showed the lowest significant values.