PROBIT estimates biologic activity by weighted probit regression analysis and comparison with a known standard. The program is intended to speed up calculation of the concentration of cytokines in large numbers of cell supernatants. Data are input from either sequential or free-form text files created with a spreadsheet or text editor. This allows transfer of data from a beta-counter equipped with a suitable terminal without retyping. Results are displayed and/or printed as relative potency (% of standard) and 50% effective dose (ED50).