The gerHABC operon of Bacillus anthracis, encoding a gerA-like family member of germinant sensors, was shown to be required for endospore germination in the presence of macrophages and in macrophage-conditioned media. The loss of the germination phenotype in macrophage cultures of B. anthracis gerH-null endospores was restored by complementation in trans with a wild-type copy of gerH expressed under the control of its own promoter. Using endospores from both the parental strain B. anthracis Sterne and an isogenic gerH-null strain, we partially characterized germinants secreted by macrophages into the extracellular medium.