An immunoassay method for detection of antigen is disclosed. The method employs complement mediated lysis of vesicles loaded with In-111 or other gamma-emitting cation, and quantitative detection of the lysis by gamma-ray perturbed angular correlation (PAC) spectroscopy. The vesicles are labeled with a substance competitive to the antigen to be measured, and the concentration of the antigen in the sample measured by assessing the diminution in lysis due to the presence of the competing antigen.The method may also be used to assess the immunologic competence of a subject by injecting suitably sensitized vesicles and monitoring the in vivo lysis pattern by (PAC).