This investigation was adopted during the two consecutive winter seasons of 2019/2020 and 2020/2021, to examine the impact of foliar spray with bread yeast in different forms and concentrations on the development, production yield and quality of lettuce cv. Dark Green (Lactuca sativa L.). This experiment was consisted of twelve treatments which were the combinations between four yeast forms (untreated yeast cells, yeast extract, yeast cells treated with nitric acid 50% and yeast cells treated with urea at three concentrations (1, 2 and 3, w/v).The results showed that, foliar spraying with yeast cells treated with urea at the highest concentration 3% recorded the largest values of the height of plant, leaf area per plant, total fresh and dry weights/plant, total chlorophyll, yield and its components, N, K and dry matter percentages in lettuce leaves in both seasons, with no significant differences between foliar spray with yeast cells treated with urea or with nitric acid treatments regarding number of leaves/plant, P (%) and nitrate content. In general, foliar spraying with yeast cells treated with urea at 3% increased yield and its component but at the same time increased nitrate content in leaves and decreased the quality of lettuce plants. Accordingly, it may be prudent to avoid using a high concentration of yeast cells treated with urea, in order to preserve the quality of lettuce plants, and instead of that it can be used a medium concentration to get the best quality and reasonable yield.