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The study evaluated the dose–response relationship of two commercial microbial biocontrol agents, <i>Bacillus subtilis</i> and <i>Gliocladium catenulatum</i>, against <i>Botrytis cinerea</i> both in vitro and in vivo. Inoculum doses, formulation, temperature and foliar leaf part all affected the control achieved by the two BCAs. In vitro competition assays on modified PDA plates tested a range of BCA doses (log₁₀ 3–10 CFUs or spores/droplet) at 4, 10 and 20 °C on the development of <i>B. cinerea</i> colonies. The dose–response relationship was influenced by both the BCA formulation and temperature. In vivo studies on lettuce plants in semi-commercial greenhouses examined the BCA dose (log₁₀ 5–9 CFUs or spores/mL) for controlling <i>B. cinerea</i> with a high inoculum (log₁₀ 6 spores/mL). Leaf disc assays showed that the dose–response relationship was influenced by the leaf parts sampled. These results suggest that the dose–response relationship between a BCA and specific pathogen will be significantly influenced by environmental conditions, formulation and plant phyllosplane tissue.