The results of a feasibility study on the automation of the micronucleus assay in whole blood cultures of human lymphocytes are reported. The assay requires determination of the number of lymphocytes with micronuclei among the proliferating population. Using an in-house-assembled image analysis system, a prototype software package was developed that addressed two problems: micronuclei identification and discrimination of nonproliferating cells from proliferating lymphocytes (the only ones that can give rise to micronuclei). The results of manual verification of automated micronucleus scoring showed that 70% of all digitized micronuclei were extracted from the images and 90% of them were correctly classified and paired with a parent nucleus by an "affinity function". The discrimination between proliferating and nonproliferating cells was carried out by linear discriminant analysis of simple nuclear features extracted from Feulgen-stained cells. Among the Feulgen-stained nuclei that were identified by autoradiography as proliferating or not, 85% were correctly classified by a six-feature discriminant function.