This paper describes a fast and accurate method for the identification of autoradiographically labelled human hypoxanthineguanine phosphoribosyl transferase (HGPRT) variant lymphocytes on slide preparations using a high-speed computer image analysis system-the Fast Interval Processor (FIP). The system has been developed primarily for the analysis of cultured human peripheral blood lymphocyte populations in which the frequency of labelled cells may vary from less than one to more than 2,000 in every 10, OOO cells. Evaluation experiments have demonstrated the excellent performance of FIP in the counting of labelled and total cells over this frequency range.