A high content of mast cells (MC) is considered characteristic of neoplasias. Some researchers postulate MC as enhancers of tumor development, others as inhibitors. The purpose of this study was to evaluate the ability of peritoneal cavity MC to modulate the in vivo and in vitro growth of two murine mammary adenocarcinomas with low (M3) and high (MM3) metastatic capacity. MC from the peritoneal cavity of normal (NMC) or tumor-bearing mice (TMC) were used. TMC, which by histochemical methods appeared degranulated, were not able to modify the tumorigenicity of both tumors. NMC, in contrast, decreased M3 tumor incidence and cell proliferation in vitro and increased the latency period of only MM3 tumors. No changes in the number of spontaneous lung metastases could be seen in experiments carried out either with NMC or TMC. We conclude that NMC, which are rich in chemical mediators, can modulate some of the first steps of tumor development. Once tumor-mediated degranulation occurs, MC become unable to regulate it.