Attachment to host cells of the respiratory epithelium by Mycoplasma pneumoniae is a complex, multicomponent process, requiring a number of accessory proteins in addition to adhesins directly involved in receptor binding. In this study, protein phosphorylation of the cytadherence-accessory proteins HMW1, HMW2, and HMW4 of M. pneumoniae was examined using biochemical and immunological techniques. The initial indication of protein modification came from Western immunoblot analysis of the two-dimensional polyacrylamide gel electrophoresis (PAGE) profile of M. pneumoniae proteins, revealing multiple spots for both HMW1 and HMW4 that varied in pI but not in size. M. pneumoniae cultured in the presence of H3(32)PO4 exhibited numerous phosphorylated proteins as detected by sodium dodecyl sulfate-PAGE and autoradiography. These included proteins corresponding to HMW1, HMW2, and HMW4 in electrophoretic mobility. The Triton X-100 partitioning characteristics of these phosphorylated proteins was identical to that described previously for HMW1, -2, and -4. Furthermore, these protein bands were absent when a noncytadhering variant deficient in HMW1-5 was examined in the same manner. Finally, the availability of antiserum to HMW1 and -4 enabled us to confirm by radioimmunoprecipitation that HMW1 and HMW4 are phosphoproteins. Phosphoamino acid analysis of acid-hydrolyzed HMW1 and HMW2 identified primarily phosphothreonine and, to a lesser extent, phosphoserine in HMW1 and predominantly phosphoserine, with a trace of phosphothreonine, in HMW2. Neither protein contained phosphotyrosine. HMW1-HMW5 are components of a cytoskeleton-like structure in M. pneumoniae that is thought to function in cell division, changes in cell morphology, gliding motility, and the localization of adhesins in the mycoplasma membrane. Phosphorylation may regulate cytoskeleton dynamics involving these cytadherence-accessory proteins.