One (1) anti-Blastocystis serum from a monkey naturally infected with isolate M12 and four (4) hyperimmune sera raised in inbred Balb/c mice against crude antigens of two Blastocystis isolates (C and KPI). one each of Entamoeba histolytica (HK9) and Giardia lamblia (7404) were used to react with several homologous and heterologous Blastocystis isolates, E. histofytica, G. lambfia, Endolimax nana and Bac-4 (Escherichia coli isolated from culture medium of a B. hominis isolate KPl). All anti-Blastocystis sera did not show cross-reactivity with E. histolytica and G. lamblia by western blotting. Similarly, anti-E. histolytica and anti-G. lamblia sera also did not react against all Blastocystis isolates tested, even though these three protozoa are known to produce diarrhoea in humans. Polyclonal sera raised against antigen prepared from xenic culture of Blastocystis produced a smear reaction on the immunoblot, while antibodies raised against antigens prepared from axenic culture (isolate C) gave prominent reaction bands. This may be due to the purity of the immunogen used in inducing the immune response. The cross-reactions of sera from mice immunised with the xenic B. hominis isolates may also due to antibodies against E. coli. Anti-Blastocystis serum from monkey's with natural infection showed several prominent reaction bands together with a smear at above 40 kD were most probably induced by the excretory-secretory antigens of the parasite. A variety of reaction patterns were obtained with these anti-sera and the antigens from different Blastocystis isolates. These may be reflects from differences in antigenic components from various strains of this parasite. KEYWORDS: lJIastocysfis, polycional antibodies, immunoblot, experimental animals