The interexperimental variation often observed following the repeated monitoring of immune phenomenon was analysed using the alloimmune cellular response that developed in the spleens of B6AF mice following a challenge with Mastocytoma P815-X2 and intervention with BCG. The response, mediated solely by T cells, could be defined in terms of the involvement of nylon-wool adherent and nonadherent T cells. The relative contribution of each cell type to the response was found to vary considerably and could be use to define the interexperimental variation. Hence when a given treatment resulted in a poor response, the major contributors were nonadherent to nylon-wool, while when the same treatment yielded a good response, adherent cells became the predominant contributors. It was concluded that the inter-experiment variations observed here were a biological event, rather than an experimental artefact, being a function of the relative contribution of different T-cell subpopulations.