The influence of subcutaneous infection with protoscoleces of Echinococcus granulosus on the distribution of lymphocyte subsets in draining lymph nodes has been evaluated by immunocytochemical labelling of lymphocyte surface antigens. These studies reveal marked expansion of paracortical Thy-1+, Lyt-1+ cells and moderate proliferation of surface immunoglobulin-bearing B-cells immediately after infection. The Lyt-1+:Lyt-2+ ratio decreases rapidly during the first 21 days post-infection and remains below unity until at least 12 weeks post-infection due to severe depletion of Lyt-1+ cells in draining lymph nodes and a significant increase in the percentage of Lyt-2+ cells. The potential role of these Lyt-1-,2+ (putative T-suppressor) cells in regulation of the anti-parasite immune response and mediation of generalized immunosuppression is discussed in the light of evidence of inhibition of anti-parasite immunity in infected mice.