A greenhouse experiment was conducted in 2011-12 to determine the effects of fertilization using an Asahi SL biostimulator on heavy metal concentrations in two cultivars of eggplants (Solanum melongena L.) grown in an unheated plastic tunnel in the garden of the Research and Experimental Station of the University of Warmia and Mazury in Olsztyn. Eggplants with their high nutritional and biological values plus popularity, eggplants are an important crop economically. However, due to their temperature requirements, eggplants are difficult to grow in regions with less favorable climatic conditions. Biostimulants and biopreparations are applied to protect plants against exogenous stressors, and to promote their healthy growth and development. Such protective treatments are increasingly applied in the cultivation of thermophilous vegetables in colder climates. The Asahi SL biostimulator supports the plant’s natural defense mechanisms, thus alleviating the negative effects of adverse environmental conditions. Under optimal growing conditions the biostimulator helps crops reach their full genetic potential. The experimental materials comprised plants of two eggplant cultivars: Black Beauty and Violetta Lunga. Seedlings were grown in line with the generally observed standards for eggplant cultivation. The heavy metal content of plant material (in fruit) was determined. The concentrations of heavy metals and nitrates (V) in eggplants grown in an unheated plastic tunnel did not exceed the maximum permissible levels. The biostimulator exerted varied effects on the heavy metal and nitrate (V) contents of eggplants, and it increased the accumulation of mercury in the edible parts of eggplants. A significant increase in the concentrations of cadmium and nitrates (V) was noted in the cv. Black Beauty control eggplants.