A laboratory incubation experiment was conducted to evaluate the effects of cadmium and lead applied alone and in various combinations on the size of the soil microbial biomass in a red soil. Treatments included the application of cadmium at six different levels i.e.,0(background), 5, 15, 30, 60 and 100 μg g^(-1) soil alongwith each of the four levels of lead (0, 100, 300 or 600 μg g^(-1) soil). Results indicated that the application of cadmium and lead alone to the soil significantly (P＜0.001) affected the soiI microbial biomass. The microbial biomass carbon (Cmic), biomass nitrogen (Nmic) and biomass phosphorus (Pmic) decreased sharply with increase in the levels of the soil applied cadmium or lead. A significant increase in the soil microbial biomass C:N ratio and decline in the microbial biomass C:P ratio was also observed due to soil applications of cadmium or lead. Cadmium and lead applied together in various combinations exhibited significantly greater biocidal effect on soil microbial biomass: than either of the cadmium or lead applied alone. The Cmic and Nmic in the tested soil significantly (P＜0.001) declined due to combined effect of cadmium and lead as compared with the same levels of cadmium or lead applied alone. A significant increase (0.001＜P＜0.05) in the biomass C:N ratio and a non-significant decline (P＞0.05) in the Pmic and the biomass C:P ratio was also observed due to combined toxicity of cadmium and lead.