Chemosensitivity assays are widely used to predict the ability of tumor cell lines to respond to potential or existing cytotoxic drugs. In this study we have compared the cell cloning assay first described by Salmon and Hamburger with a recently developed assay which measures viable cell number by ATP luminescence. Methotrexate (MTX) was chosen as the test agent, since cell lines with varying degrees of sensitivity to this agent were readily available. The results shown good correlation between the two assays, both of which are able to discriminate between the various cell lines used. MTX inhibition of primary breast carcinomas and cell lines shows a steep dose-response curve with a threshold concentration above which increasing dose does not increase sensitivity. In solid tumors, the plateau is usually reached at a level well below 100% inhibition. The ATP luminescence assay allows discrimination of MTX sensitivity between breast carcinomas and has considerable technical advantages over the cloning assay.