It is well established that human diseases associated with abnormal immune function, including some common infectious diseases and asthma, are considerably more prevalent at younger ages. The immune system continues to mature after birth, and functional immaturity accounts for much of the increased susceptibility in the young. Although not established absolutely, it is generally believed that development constitutes a period of increased immune system susceptibility to xenobiotics, since adverse effects may occur at lower doses and/or immunomodulation may be more persistent, thus increasing the relative risk of xenobiotic exposure to the immunologically immature organism. Data from published reports were compared to determine whether age and developmental stage at exposure influence the immunotoxic effects of diethylstilbestrol (DES), diazepam (DZP), lead (Pb), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and tributyltin oxide (TBTO). These compounds were chosen for comparison based on the fact that each had been studied fairly extensively, resulting in a significant number of peer-reviewed publications. Based on lowest-observed-adverse-effect level (LOAEL) values for all five compounds, the developing immune system was found to be at greater risk than that of the adult, either because lower doses induced immunotoxicity, adverse effects were more persistent, or adverse effects were reported following developmental, but not adult, exposure.