The invention described herein relates to a method for identifying antigen fragments of Toxoplasma gondii proteins, and their use as diagnostic and immunogenic agents. Said method is implemented by means of selection of DNA fragments libraries of the parasite with sera of subjects who have been infected, using the phage display technique, and is characterised in that it uses the expression/exposure vector .lamda.KM4. The method allows also to identify antigen fragments related to the time of the infection.