Nisin, a bacteriocin produced by strains of Lactococcus lactis, has a broad inhibitory effectagainst gram-positive bacteria.This study investigated the efficacy of nisin Z against Lactobacillus sakei when produced by a nisin-producing strainLactococcus lactis in model cheeses. Three different UF model cheeses, containing 0, 4 or 10% of gelatin in their drymatter were inoculated with both strains. Measurement of Lb. sakei loss of viability was an indirect indicator of nisinin situ efficacy when gelatin was incorporated. After 24 h, the loss of viability of Lb. sakei was from 0.73 ± 0.14 to3.30± 0.60 log10 cfu g-1 in the cheeses with 0 and 10% of gelatin, respectively, indicating a better in situ efficacy ofnisin. However, the concentration of nisin produced by Lactococcus was similar (3.5 ?g g-1) in all model cheeses whenmeasured using an enzyme-linked immune sorbent assay (ELISA). The growth of Lactococcus was slightly improvedwhen gelatin was incorporated, leading to a higher lactate concentration, which is one of the factors explaining theincreased nisin efficacy. This study is the first one to demonstrate that the concentration of nisin does not reflect its insitu efficacy in complex environment such as cheese