Monday, 10 September 2012, 14.15 -16.15 Objective: We tried to interpret "the job" of MF in different stages of tumor growth by monitoring the dynamics of their distribution. Method: On a rat model of experimental tumorigenesis using BP6 fibrosarcoma cells injected intraperitoneally, we performed a semiquantitative analysis of the recruitment of MF in the tumor compartment (microenvironment) as well as in peripheral blood. Results: We observed a slow increase of MF count in the initial phase of tumor growth followed by a slow decrease in the late phase within the tumor compartment. Simultaneously peripheral blood showed constant increase throughout the experiment resulting in typical leukocytosis accompanying developed tumors. Conclusion: Increased MF in the initial phase could reflect their enrollment to "prepare a suitable microenvironment for tumor seeding". Their decrease began after the tumor size was macroscopic, thus it was time for them to carry out a "different job". In the mean time their amount in peripheral blood started becoming significant, this is also the time when tumor -host interactions reach a systemic level. In futher experiments we will attempt to verify our assumption: When the tumor flourish in the host organism, it uses informed, transformed and recruited macrophages for growth and building construction of the tumor, and for communication with distant organs on a regulatory level.