Abstract: The present invention relates generally to immunogens and their use in vaccine preparations. More particularly, the present invention is directed to a peptide or polypeptide or a derivative, homologue or analogue thereof which corresponds to, mimics, or cross-reacts with, B-cell or T-cell epitopes on surface polypeptides encoded by Mycoplasma pneumoniae or M. genitalium. The immunogens of the present invention are particularly useful in vaccine preparations for the prophylactic and therapeutic treatment of individuals against infections by Mycoplasma ssp. The present invention further provides diagnostic reagents for the detection of Mycoplasma ssp. in biological samples derived from individuals suspected of being infected therewith.