A simple procedure for extracting a lectin from the seeds of the jackfruit, Artocarpus integrifolia, is described. The extracts had an average protein concentration of 13.3 mg/ml, and were strongly erythroagglutinating with a 10(6) titre. Erythroagglutination was shown to be non-specific for A1-B-0 erythrocytes. Lymphoblastic transformation and mitogenic stimulation occurred in lymphocyte cultures both in man and Rhesus monkey, and chromosome preparations were obtained. Such preparations were of good quality and were adequate for chromosome analysis with chromosome banding techniques. In man, the response to the mitogenic stimulation did not vary significantly between individuals. In Rhesus monkey, where Phaseolus vulgaris phytohaemagglutinin produced a weaker mitogenic stimulation than in man, jacalin proved to be very effective.