117 patients with breast cancer were subjected to a comparative study for immune complexes by the polyethylene glycol precipitation method, T-lymphocytes by the E-rosette test, serum immunoglobulins by immunodiffusion as well as for lymphocyte blast transformation in response to PHA. The level and incidence of immune complexes were elevated. An increase in the tumour mass with stages is associated with high levels of circulating immune complexes. No relationship was found between morphology of the tumour and the level or incidence of immune complexes. In latest stages (III-IV) the level of immune complexes correlated significantly with depletion of T lymphocytes or their response to PHA, and also with concentration of serum IgM. The results suggest that immune complexes may function as a specific blocking factor in the course of oncological disease.