Quantitative and qualitative precipitin tests using antibodies prepared against specific carrageenans from Chondrus crispus were performed on carrageenans that were extracted from plants of known stages in the life cycle of various other species of red algae. Species from three genera in the family Gigartinaceae were studied and it was determined that specific anti-κ-carrageenan serum was precipitated by carrageenans that were extracted from gametophytic plants but not by carrageenans from tetrasporic plants. Carrageenans and related polysaccharides from several species outside of the family did not show this difference.KCl-soluble carrageenan from female plants precipitated less antibody than KCl-insoluble carrageenan, but the soluble fraction that was treated with alkaline borohydride precipitated as much antibody as the insoluble fraction. Alkaline-borohydride-treated λ-carrageenan also precipitated the anti-κ-carrageenan serum. Other polysaccharides that precipitated anti-κ-carrageenan serum include iota carrageenan and furcellaran.Evidence presented from the above precipitin reactions indicates that 3,6-anhydrogalactose is involved in the determinant site of the antigen.