The growth and development of plants in natural or formed ecosystems is frequently modified by physical and chemical processes due to the influence of neighboring plants. The present studies were aimed at describing the allelopathic interspecies interactions in mixture crops. It is assumed that dominance of a certain crop species in a field may be explained by their strong allelopathic potential against the neighboring species. The preliminary inter-crop influences were researched in laboratory of tests performed on Petri dishes. These tests allowed determination of allelopathic effects of tested grain species on germination energy and the germinating ability of seeds of other grain crop species as well as on the length of root and aerial parts of their seedlings. Four grain crop species were tested, including barley, oat, wheat and triticale. Results of the experiments showed that some crop species when grown in a mixture with other crops affect their germination energy and seedling growth.