We tested the hypothesis that popliteal lymph node assay (PLNA) in the modification of graft vs host reaction (GvHR) enabled to assess immunosuppressive potential of xenobiotics. We conducted PLNA in GvHR modification under two experimental conditions using cyclophosphamide (CY). In the first experiment average lymph node weight index (IW, weight ratio of popliteal lymph node of hind footpad injected with parental lymphocytes to lymph node of footpad injected with vehicle) in rats administered intraperitoneally with CY simultaneously with, or 3 days before local GvHR induction was 84% and 42%, respectively less than average IW in external control animals injected with parental lymphocytes into one footpad and vehicle into contralateral footpad. Average lymph node lymphocyte index (IL, ratio of cell number in the lymph node undergoing GvHR to control lymph node) in the tested animals was 92% and 86%, respectively less than in controls. In the second experiment in rats injected subcutaneously into one footpad with parental lymphocytes suspended in solution of CY at the concentrations of 4.6 mM or 18.6 mM and with parental lymphocytes into contralateral footpad, average IW was 88%and 92%, respectively less than average IW in the external control animals injected with parental lymphocytes into both footpads (average IL was 84 and 91%, respectively less than in controls). Our preliminary experiments showed a strong inhibitory effect of CY in both PLNA-GvHR models used and we believe that both models may be of value in designing future protocols aiming at prediction of immunosuppressive potential of chemicals.