Four surface proteins of the phytopathogenic MollicuteSpiroplasma citriwere characterized using monospecific polyclonal antisera. Antiserum specific for P29 (spiralin) caused growth inhibition, spiral deformation, and metabolism inhibition, while anti-P89, anti-P77, anti-P58, and preimmune sera did not. In Triton X-114 phase partitioning, P29, P89, and one of two P77 bands were hydrophobic, indicating the probability that they are integral membrane proteins. P58 partitioned into the hydrophilic phase and may be an extrinsic membrane protein. A second band which reacts with anti-P77 serum was also hydrophilic. Treatment of the two P77 components with trypsin and chymotrypsin resulted in very different polypeptide patterns, suggesting that the two bands represent unrelated proteins. These results demonstrate the variability in hydrophobicity among surface-exposed proteins inS.citriand describe procedures that should prove useful for characterizing and separating other proteins of interest in spiroplasmas.Key words:Spiroplasma, mollicute, surface proteins, serology, Triton X-114.