The purpose of the present work was to study the in vitro effects of challenging human whole-blood and platelet-rich plasma (PRP) with neoplastic cells (NC) isolated from a murine mammary gland adenocarcinoma. Viable, but not formalinized, isolated tumoral cells induced platelet aggregation in citrated whole blood. NC did not induce aggregation of PRP samples whereas the NC added to PRP induced aggregation in 33% of volunteers if samples were previously stimulated with a subaggregatory strength of collagen. The results suggest a role for other blood cells since the proaggregatory effect was less evident on PRP. Adenocarcinoma M3 seems to be a suitable model to study in vitro platelet-cell tumor interactions.