HAYSTAQ is a comprehensive computer system for searching chemical information and is particularly directed toward the stringent requirements of the U. S. Patent Office. The greatest activity to date has been in the design of a satisfactory method to search for chemical structures. A structure diagram is considered as a network, where the atoms or functional groups are the nodes and the bonds between them the links. The search algorithm consists of attempting to match, via a topological tracing, a question network (structure) against each structure in the file of chemical compounds stored on magnetic tape. The structure search includes provision for Markush structures and other generic concepts. Each of 385 questions was matched against a file of 2,400 entries containing (because of the Markush feature) effectively 162,000 compounds. The continuation of this work includes the use of the Hayward linear notation as input and extension of the search routines to other kinds of information associated with chemical structures. (Author).