The data of hygienic monitoring of the Saint Petersburg environment suggest that the soil of the megapolis is greatly polluted with heavy metals among which lead is of priority. The studies performed indicate that under the environmental and hygienic conditions, the level of lead in the hair of children is a qualitative and quantitative criterion for the negative impact of chemical pollution of the environment. The threshold level at which there are higher morbidity rates in children is 5.8 micrograms/g. The findings suggest that there is a reduction in the earlier accepted critical level 8 micrograms/g. (V. Lukovenko, 1990; B.A. Revich, 1999). When its level is 8 micrograms/g, the children are found to have retarded mental and physical development. The results of examination of the nonspecific resistance system in children living under the conditions of the megapolis show that the level of nonspecific defense decreases when the hair content of lead is 5 micrograms/g.