Lymphocyte proliferation is a widely used technique to assess immune competence. However, the technique is subject to a large degree of variation, some biological and some technical. In this study, the components of variation in whole blood proliferation assays were analysed over time, using both antibody and mitogenic stimulants. The levels of variation within individual samples, between individuals and between groups of individuals over time were examined. A method of transforming the data is proposed which reduces the coefficients of variation to an acceptable level, and which expresses individual results as a standardised count. This method overcomes the problem of different levels of absolute counts, it corrects for time sensitive errors and allows data from multiple laboratories to be pooled.