Cells of Oocystis solitaria after treatment with inhibitors of a) microfibril orientation b) microfibril synthesis, and c) microfibril crystallization have been investigated with the freeze-fracturing technique. Changes in morphology of the E-face of the plasma membrane as a result of these treatments have been recorded. Terminal complexes are removed by cycloheximide, but not by colchicine or congo red treatments. Microfibril imprints are removed by congo red but not by colchicine or cycloheximide treatments. Congo red induces in addition the presence of increased numbers of paired, thickened, terminal complexes. Prolonged (4-6 h) treatment with congo red causes the insertion of a second set (oriented at 90 degrees to the first set) of parallel terminal complexes in the E-face. These results are discussed in relation to plasma membrane turnover.