IL-2 activity is commonly estimated in cell culture supernatants by an IL-2-dependent cell proliferation assay. This method is both reproducible and sensitive. However, it often appears from the literature that statistical analysis of the titration data either is disregarded or, when performed, is based on statistically incorrect assumptions. The proposed method is based on the principle of biological assay by parallel lines as applied to probit analysis of quantitative responses. The procedure has been embodied in a simple and interactive computer program which automatically estimates the IL-2 concentration in the biological sample, in terms of U/ml, and provides its standard error and confidence limits. This program is also suitable for quantitative determination of other biologically active substances that show a sigmoid dose/response relationship.