The interest in organic production of vegetable has raised in the last decades. To start an organic field production of vegetables the seeds or transplant used should be obtained with organic production systems. Even though agronomic practices for organic production of vegetable have been largely investigated, there is still the need to increase information on organic transplant production. Organic liquid fertilizers are available for transplant fertigation but their efficacy and rates for use have not been evaluated. Moreover, plant growth-promoting rhizobacteria have been applied to different vegetable crops but there is still no information on the combined effect of microbial biostimulant application and organic fertigation on organic production of lettuce seedlings. This study aimed to assess the efficacy of microbial biostimulants to enhance growth and quality of lettuce seedlings fertigated with increasing rates of an organic liquid fertilizer. Lettuce seedlings were inoculated with 1.5 g L⁻¹ of TNC Bactorr^S13 (containing 1.3 × 10⁸ CFU g⁻¹ of Bacillus spp.) or with 0.75 g L⁻¹ of Flortis Micorrize (containing 30% of Glomus spp., 1.24 × 10⁸ CFU g⁻¹ of Agrobacterium radiobacter, Bacillus subtilis, Streptomyces spp. and 3 × 10⁵ CFU g⁻¹ of Trichoderma spp.) and fertigated with a nutrient solution containing 0, 7, 14, and 28 ml L⁻¹ of an organic liquid fertilizer (NK 3-4). Organic fertigation had no effect or even a negative effect with the highest dose (28 ml L⁻¹) on some morpho-physiological parameters (seedling height, leaf area, SLA, NUE) of uninoculated lettuce seedlings. The use of the microbial biostimulants modified seedling growth and its response to organic liquid fertilizer levels. They had a growth-promoting effect on the unfertilized seedlings and modified the response of lettuce seedlings to organic fertigation but to different extents for TNC Bactorr^S13 and Flortis Micorizze. The inoculation of the substrate with microbial biostimulants could improve organic lettuce transplant quality whereas the fertigation with organic liquid fertilizer was ineffective or negative.