An improvement in the sensitivity of the RIA kit of cyclic guanosine monophosphate by an acetylation process before use is reported. Employing this process the binding rate of antigen and antibody was obviously increased. Thus the required amount of antibody was saved by 2 to 3 times. A method for more efficient utilization of the kit by protecting the antibody with bovine serum albumin solution and by controlling the temperature of freezing-thawing is also described.