Résumé : The influence of the micrococcal vaccine on the proliferation of normal immunocompetent and neoplastic plasma (MOPC 173) and of carcinoma (Ehrlich ascites) cells is studied. BALB c mice and CDF1 mice, hypervaccinated with Micrococcus Lysodeikticus, developed significantly (0.001 < P < 0.005) lower primary immune responses against sheep erythrocytes and also displayed impaired secondary immune responses. Administration of Bacillus Calmette-Guérin could partially restore the immunologic potential of vaccinated mice as measured by the capability of mice to answer with primary or secondary immune responses to sheep erythrocytes but did not increase immunoresponsiveness of normal mice sensitized at nearly optimal antigenic dose. Switching on a secondary antimicrococcus immune response after grafting 200,000 Ehrlich carcinoma cells in the peritoneum of BALB c mice resulted in a 30% increase in mean life span over control mice. However, mice hyperimmunized for months with Micrococcus lysodeikticus were offered the strongest immunoprotection and we found 67% of mice surviving to day 90 whereas the other vaccinated but tumour bearing animals showed a 29% increase in life span over control mice. Similarly, we demonstrated that 44% of BALB c mice, hyperimmunized for months, were able to reject a tumour challenge of 200,000 grafted MOPC 173 plasmacytoma cells, whereas the other micrococcus vaccinated but tumour bearing animals died with a 26% increase in mean survival time over control mice. © 1978.