Heterosis is a complex biological phenomenon. Because of the complex interaction and interrelation between "genes - metabolism - environment", it is hardly possible to expect a clarification of the heterosis phenomenon through simple genetic explanations only (Hagemann et al. 1967).We have followed an immunochemical aspect and method of research. The antigenic analysis of inbred lines and their hybrids was used for studying heterosis and for investigating the possibilities for its prognosis.The heterosis effect was proved under field conditions. Our investigations (Dimitrov et al. 1972) show the presence of four protein fractions in the seed extracts of inbred maize lines, while the heterotic hybrids contain a fifth protein fraction. Antigenie analysis by the method of Grabar and Williams was carried out for a more complete characterization and determination of the specificity of the fractions obtained (Grabar and Williams 1955).The present publication is a result of our research upon maize inbred lines, simple heterotic and nonheterotic hybrids and their backcrosses. A double diffusion in agar gel, according to the Ouchterlony method (Ouchterlony 1958), confirmed the presence of a fifth protein fraction in the heterotic hybrids, which can not be found in the inbred lines or in the nonheterotic hybrids.In the inbred lines we found 3 protein fractions common to all of them, and also a fourth (individual antigen), contained only in the inbred lines that produce a heterosis effect when crossed. It was determined that the carrier of the information for the synthesis of the individual antigen is a nuclear factor.We also determined the conditions under which (after direct and inverse crosses and after crosses in one direction only) heterotic hybrids are obtained. Some backcrosses show a marked heterosis effect connected with the doubling of the factor, carrier of the information for the individual antigen. This fact is important for the scientific verification of the methods for obtaining complex heterotic hybrids.Our results throw some light upon the genetic nature of the heterosis phenomenon.The heterosis effect was determined only for inbred lines whose seed extracts have a precipitation arc against their homologous serum, absorbed with the extract of its partner. This allows for the prognosis of heterosis in maize, i.e. for the determination in advance (through the double immunodiffusion) of the inbred lines and also of the direction of crosses that produce the heterosis effect.The present publication is a continuation of our report (Dimitrov et al. 1972) concerning the research on the nature of heterosis in connection with its prognosis. In our investigation we have accepted as heterotic only hybrids whose seed yield exceeds the seed yields of each of the separate inbred lines being crossed.