An immunological method was developed in this study to quantify reproductive output of the female butter clam, Saxidomus purpuratus. A clam egg-specific polyclonal antibody was developed using the purified butter clam egg as an antigen. An indirect Enzyme-Linked Immunosorbent Assay (ELISA) was used in quantitative measurement of the eggs. Size of the butter clam eggs ranged from $70.81{pm}7.52{mu}m$ in histology or $88.56{pm}11.31{mu}m$ in intact eggs. The predominant egg constituent was protein (37.44%), followed by lipids (11.40%) and carbohydrates (9.68%). The SDS-PAGE showed that the egg proteins are composed of several peptides of molecular weights consisting of 247, 200, 99, 91, 54 and 47 kDa. ELISA indicated that the clams collected from Geoje Island in May 2002 produced 8.2 to 26.8% of their body weight as eggs or 9,307,309 to 31,156,333 with a mean of 16,931,893 eggs per individual clam. The results of this study thus suggest that indirect ELISA using rabbit anti-clam egg IgG as a primary antibody is a rapid, affordable and sensitive method to assess reproductive output of 5. purpuratus and possibly other bivalves using a small amount of eggs.