The goal of this work was to determine: a) do lyophilized human melanoma BG or Fem-X cells affect the proliferative capacity of normal human peripheral blood mononuclear cells (PBMC) and b) does the PBMC six-days preincubation in nutrient medium with FBS with, or without lyophilized human melanoma BG or Fem-x cells, affect their suppressive action on the survival of the same malignant cell line in vitro. In the aim to avoid any stimulating effect of FBS, other group of experiments were done in nutrient medium with human AB serum in order to determine: c) does the PBMC six-day-preincubation with lyophilized human melanoma BG or Fem-x cells affect their antiproliferative action on the corresponding malignant cell line in vitro and d) does the PBMC six-day preincubation with lyophilized normal PBMC, obtained from healthy volunteer (as a source of allogenous, but not of tumor antigens), affect their suppressive action on the survival of both melanoma BG and Fern-x cell lines in vitro. Results obtained in the presence of FBS in nutrient medium, showed that lyophilized BG cells induced a proliferation of the healthy PBMC, depending on the number of stimulating lyophilized cells. Lyophilized Fem-x cells induced healthy PBMC proliferation in lesser degree than lyophilized BG cells. This stimulation was almost constant, not dependent on the number of stimulating lyophilized Fem-x cells. Six-day stimulation in vitro by both lyophilized melanoma cells enhanced the suppressive action of PBMC on the survival of the corresponding malignant cell line. Experiments done in nutrient medium with normal human AB serum showed that six-day stimulation with lyophilized melanoma cells enhanced, again, the suppressive action of PBMC on the survival of the corresponding malignant cell line. Contrary, six day preincubation of normal PBMC with the lyophilized healthy PBMC (obtained from other healthy person) inhibited their suppressive action on the survival of both malignant cell lines in vitro.