It is known that when actomyosin and adenosine triphosphate (ATP) interact, the ultraviolet absorption is no longer that characteristic either of actomyosin or of ATP. This report deals with a spectrophotometric study of interactions of various chemicals and/or drugs with actomyosin. The intention was to discover whether these agents alter the absorption, i.e., optical density (O.D.) of actomyosin solutions and if so, whether all the agents produce similar changes.