1,25-dihydroxyvitamin D3, the active metabolite of vitamin D, partially inhibits antigen and mitogen-driven lymphocyte stimulation. We studied the effect of granulomatous inflammation on the sensitivity of lymphocytes to 1,25-dihydroxyvitamin D3 in vitro, measuring the inhibitory effect of 1,25-dihydroxyvitamin D3 on mitogenesis of splenocytes of mice with chronic inflammation induced by subcutaneous injection of talc. Systematic manifestations of the local inflammation included loss in body weight, splenomegaly, enhanced DNA synthesis by freshly isolated splenocytes and enhanced prostaglandin secretion by activated splenocytes. Splenocytes from animals with local inflammation were more susceptible to inhibition by 1,25-dihydroxyvitamin D3, but not by prosaglandin E2. This increased sensitivity to 1,25-dihydroxyvitamin D3 was abolished by blocking prostaglandin synthesis in splenocyte cultures with indomethacin and was restored by adding prostaglandin E2. This effect cannot be attributed to enhanced prostaglandin synthesis in the presence of 1,25-dihydroxyvitamin D3, but is probably due to a qualitative change in the response of splenocytes from inflamed animals to the combined action of 1,25-dihydroxyvitamin D3 and prostaglandin E2.