The human body has a certain capacity to withstand potentially adverse effects of environmental exposures. Chemical exposures may result in a weakening of the body defenses, i.e. a decrease in reserve capacity, thus leading to increased susceptibility to other exposures. Thus, despite normal hematological findings in the initial examination of a group of lead-exposed workers, the lead exposure caused a decreased reserve capacity for blood formation; these effects became prominent only after phlebotomy where the lead-exposed workers showed a decreased rate of blood regeneration. Evidence from studies of cell attrition and age-related functional decreases of the nervous system and the kidneys suggest that a reserve capacity exists and that it may be vulnerable to chemical exposures. Reserve capacity may therefore be a notion of wide significance with regard to chemical exposures as part of multicausal disease etiologies.