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The experiments reported in this paper examine the cell-surface phenotype (CD5, CD4, CD8, TI9, MHC class II and sIg) and cell output of lymphocyte subsets circulating through a subcutaneous lymph node in the sheep fetus, in an environment unaffected by foreign antigen and circulating immunoglobulins. CD4+ lymphocytes were the major T-cell subset in fetal lymph and were clearly enriched in lymph compared with blood, whereas TI9+, CD8+ and B lymphocytes were not. It seems likely that in the fetus CD4+ lymphocytes are extracted from the blood at a faster rate than are other T-cell subsets and B cells. There was a much higher percentage of CD8+ and T null cells and a lower percentage of MHC class II+ and B cells circulating in the fetal lymph than in adult lymph, while the percentage of TI9+ lymphocytes in fetal blood was twice that in the adult. Although the hourly cell output from an adult prescapular lymph node was far higher than that from a fetal lymph node, the circulation of lymphocytes through fetal lymph nodes was much greater per gram lymph node weight than that through adult lymph nodes. The wholesale recirculation in the fetus of all the major T-cell subsets found in the adult is paradoxical because it is not known what function they serve in the fetus in the absence of antigen and ongoing immune responses, although clearly they are not memory cells. [ABSTRACT FROM AUTHOR]