We developed a polyclonal antibody from egg proteins of the butter clam, Saxidomus purpuratus, to quantify eggs using an enzyme-linked immunosorbent assay (ELISA). SDS-PAGE showed that the egg protein was composed of several peptides with molecular weights of 247, 200, 99, 91, 54 and 47 kDa. An immunoblotting assay indicated that the egg-specific antibodies were developed from the 200 and 99 kDa peptides. The rabbit anticlam egg IgG was able to detect as little as 0.078 μg/mL of S. purpuratus egg protein by ELISA. A weight-normalized gonadosomatic index (GSI, mg dry egg/mg dry tissue) was determined to follow the monthly changes in egg production of the clams. Clam-egg protein could be detected by ELISA during the entire sampling period (December to July), and GSI ranged from 0.035 (December) to 0.156 (February). The fecundity of the clams was measured from two spawning peaks in February and May and was estimated to be 22.6 million eggs (GSI of 0.16) in February and 16 million eggs in May (GSI of 0.15). The indirect ELISA used in this study was rapid, affordable and sensitive enough to assess minute amounts of egg protein, which is difficult to measure using traditional methods such as induced spawning or histology.