The number of bacteria needed to cause the hypersensitive (HR) or other necrotic responses in plant leaves was determined often by the environmental conditions under which the plant was grown prior to and after intromission of the bacteria into the intercellular spaces. Low temperatures reduced the number of Pseudomonas syringae, P. phaseolicola, and P. savastanoi cells required for HR whereas they increased the cell numbers required with P. solanacearum. Temperature and the tobacco species used affected induction of HR by xanthomonads. The amount of light influenced the type of response induced by agrobacteria with a necrotic response occurring under conditions of darkness.