A western blot assay to detect antibodies against Sarcocystis aucheniae (SA) from alpacas was developed for in vivo diagnosis of sarcocystiosis in domestic camelids. The antigen consists of a soluble extract obtained from SA macrocysts (MSA) of naturally infected alpacas. A SDS-PAGE (15%) electrophoretic analysis revealed fifteen peptides at 25-127 kDa. Western blot assay ofsera from MSA immunized rabbits detected three peptides at 58-50 KDa, while the sera from SA diagnosed alpacas reacted against four protein bands. Sera from neonate alpacas and from non-SA infected alpacas did not display reactivity to the MSA peptides. Specific immune reactivity ofthe alpaca antibodies against SA with four MSA peptides by a western blot assay has been documented.