The advances in recombinant DNA technology in recent years have had a dramatic effect on the area of protein crystallization. Large amounts of pure protein produced in various expression systems have made it possible to conduct experiments that would have been impossible with material from natural sources. With many more laboratories becoming involved in crystallizing proteins a great deal of new information has been generated on techniques to eliminate the so called 'bottleneck of crystallization' in determining a three-dimensional protein structure. More and more new and interesting proteins are being submitted to this laboratory for crystallization. Certain criteria may be set before crystallization trials are started, such as solubility, purity and aggregation tendencies. The introduction of robots now facilitates the screening of crystallization conditions. In cases where no crystals have been obtained after initial screening it can now be decided which possible modifications can be made to the protein itself to improve the chances of obtaining crystals.