A rapid microassay for hemolytic antibody using an automatic spectrophotometer was developed. This microassay was parallel to hemolytic plaque forming cell (PFC) assay most commonly used to detect antibody formation. The former had more advantages than the latter did. The data obtained using arsenic and some metallic compounds as immunomodifiers suggested that this microassay was suitable for the screening of immunomodifiers in immunotoxicological and immunopharmacological studies which deal with a large number of samples within limiteid time.