A rapid filter paper dot-immunobinding assay was adapted to detect the wall-less mollicute Spiroplasma citri in medium, plants, or insects. Filter paper spotted with sample was incubated first in dilute antiserum, then in protein A-peroxidase, and finally in a substrate of 4-chloro-1-naphthol plus hydrogen peroxide. The detection limit averaged 2.3 x 10 CFU/ml in cultures, and S. citri was detected in single infected leafhoppers. This assay was less sensitive but more rapid and economical than an enzyme-linked immunosorbent assay.