Controlling the size and quantity of organelles through competition for a limited supply of components is rapidly emerging as an important cellular regulatory mechanism. Cells contain multiple F-actin networks that are constructed by specific assembly factors. Activation of assembly factors at a specific time and place was thought to be the primary mechanism that controls F-actin network size and density. However, we are investigating an additional hypothesis that competition between actin assembly factors for a limiting pool of actin monomers also regulates networks. We discovered that F-actin networks are in homeostasis, wherein depletion of one network leads to an increase in the others (Figure A). The abundant small G-actin binding protein profilin is a key molecular component of the mechanism that ensures the correct distribution of G-actin between rival assembly factors. Profilin promotes formin-mediated actin assembly, but inhibits Arp2/3 complex-mediated actin assembly (Figure B). We are using a combination of live cell confocal microscopy and in vitro Total Internal Reflection Fluorescence (TIRF) microscopy approaches to study the mechanisms by which this competition affects F-actin network density and dynamics.