The passive immunization of pregnant female rats to S-100 protein often leads to ultra-structural abnormalities in the brain glial structures of the offspring of these rats and induces signs of delayed development in the fetal brain. Additionally passive immunization of pregnant animals with certain antigens induces permanent Ag-specific changes in the immune response of their offspring. The purpose of this study was to investigate serum immunoreactiviy (SIR) to S-100 in cerebral-palsied and developmentally-delayed children as well as in their healthy parents and to evaluate its significance related to radiologic findings of brain MRI and single photon emission computed tomography (SPECT). The subjects were children with cerebral palsy and delayed development that had abnormal findings on brain MRI or Brain SPECT. SIR to S-100 protein was measured by ELISA method in the patients, their healthy parents, 20 normal adult controls and 22 normally developed children. The SIR to S-100 protein was significantly higher in the cerebral-palsied and developmentally-delayed children when compared to that of the normal control group children. Increased SIRs were detected in healthy mothers but not in their fathers. There was no difference of SIR between the cerebral-palsied and developmentally-delayed children or any significant difference of SIRs according to the findings of the brain MRI or to developmental quotients. But, the SIRs to S-100 protein were higher in the group of more abnormal findings on brain SPECT.