Prostaglandins (PGs) have been implicated as possible modulators of the immune response and various inflammatory processes^1–4. Various cell components of the immune system are sources of PGs, and mitogen or antigen stimulation of human or murine lymphocytes leads to their enhanced release^5–8. They are also released from various human and animal tumours^9,10. Thus, as cells of the immune system are both sensitive to and sources of PGs, these factors may be important as physiological immune regulators. For example, PGs of the E series are capable of inhibiting many effector functions^9–15. They have also been shown to inhibit the development of the humoral response^16,17. Although they inhibit the proliferative response to mitogens^18,19, little is known about their effects on the development of the cell-mediated immune response to antigens. The data summarized here implicate PGs, thromboxane A₂ (TXA₂) and prostacyclin (PGI₂) in the regulation of cellular immune responses at the inductive phase. Some of these data have been reported in abstract form²⁰.