Leachates from landfills constitute a very complex environmental matrix with potentially toxic properties for both the environment and humans. Due to this fact, toxicological tests were carried out on landfill leachate (LL) obtained from the local landfill in Hryniewicze (Poland). The analyses included physicochemical studies of leachates and their impact on soil and plants, and studies conducted in bacterial models and human cell models. The results obtained indicate high contents of metals and organic matter, especially dangerous PAHs, in the tested leachate. This results in the influence of LL on changes in the content of assimilation pigments and oxidative stress observed in plants grown in soil fed with leachates. The effect of leachate on the growth of Sporosarcina pasteurii, Staphylococcus aureus, Lactobacillus rhamnosus, Saccharomyces boulardii and Candida albicans varied depending on the strain and LL dose. A particularly significant increase in proliferation after exposure to LL was noted for S. aureus. In studies conducted on human cancer cell lines representing three types of glioblastomas and one type of colorectal adenocarcinoma, a particularly significant increase in the viability of cells treated with LL was noted for the DLD-1 cell line. The results obtained, especially the stimulation of the growth of cancer cells and an increase in the number of pathogenic bacteria, indicate the potential toxic properties of the tested leachates. This is confirmed by the high level of oxidative stress in plants. The results indicate the need for continuous monitoring of waste landfills and leachates generated there.