Many biological pathways were first uncovered by identifying mutants with visible phenotypes and by scoring every sample in a screen via tedious and subjective visual inspection. Now, automated image analysis can effectively score many phenotypes. In practical application, customizing an image-analysis algorithm or finding a sufficient number of example cells to train a machine learning algorithm can be infeasible, particularly when positive control samples are not available and the phenotype of interest is rare. Here we present a supervised machine learning approach that uses iterative feedback to readily score multiple subtle and complex morphological phenotypes in high-throughput, image-based screens. First, automated cytological profiling extracts hundreds of numerical descriptors for every cell in every image. Next, the researcher generates a rule (i.e., classifier) to recognize cells with a phenotype of interest during a short, interactive training session using iterative feedback. Finally, all of the cells in the experiment are automatically classified and each sample is scored based on the presence of cells displaying the phenotype. By using this approach, we successfully scored images in RNA interference screens in 2 organisms for the prevalence of 15 diverse cellular morphologies, some of which were previously intractable.high-content screening ͉ high-throughput image analysis ͉ phenotype T he history of biology has been dramatically shaped by classic visual screens in model organisms, including Drosophila melanogaster (1-3), Saccharomyces cerevisiae (4), Caenorhabditis elegans (5), and the zebrafish Danio rerio (6, 7). In each case, biological pathways were discovered because researchers were intrigued by groups of peculiar-looking mutants and identified the genes underlying their phenotypes. Because researchers have favored the extensive study of relatively few genes (8), classic, wide-net approaches like screening are as relevant as ever to probe known biological pathways and discover new ones. Modern technology now enables large-scale experiments in cultured cells to identify human genes that underlie biological processes via RNAi. Automation also allows the screening of chemical libraries to identify perturbants useful as research tools or drugs.Despite these advances, scoring cells in images for rare and unusual morphologies has, in general, remained a significant bottleneck (9-12). Cell image analysis allows accurate identification and measurement of cells' features, enabling automated analysis of certain phenotypes that were previously intractable (13-26). However, many interesting phenotypes require the assessment of several measured features of cells. Machine learning methods that select and combine multiple features for automated cell classification have been used to score many phenotypes (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26). These methods require the provision of example cells that do and do not display the morphology of interest (i.e., positive and negative cells). Finding posi...