The ~~~d~n~ aftMy of the pr~~~~ of the small subunit of tihul~-1,5-bispho~bate ea&oxylase (pSS) to isolated, intact chlompiasts and to isolated chloroplast envelopes from the green alga C!~lunq~/a~~onas r&&z& was studied under conditions where na, import into ~~o~o~!~~ occurred. pSS bound to both chloroplasts and envelopes with equally high affinity, The dissociation constants were 5.9 & Z.IXIU~ M and 2,9 + I .4x IO-' M. res~tiv~Iy~ The tlumber of binding sires per th~o~op~t was determined to be 8.1 4 4.1~100. Binding of pSS to isolated envelopes or intact chloroplasts was specific with respect to the type of the membrane and the presence of the transit sequence.