The developing immune system may be especially vulnerable to the toxic effects of immunotoxicants. TCDD, a notorious immunotoxicant, has been shown to produce such effects in rodents as well as in man, and may be considered a prototype developmental immunotoxicant. A number of other chemicals have been identified that have been shown to affect the developing immune system. Yet, so far there are no immune parameters included in current OECD guidelines for developmental or reproductive toxicity testing. It is recommended that such tests are amended so that also the developing immune system is considered as a potential target of toxicity during developmental stages.