Lichen phycobionts are known to undergo a series of morphological, physiological, and biochemical changes as a result of lichenization. A number of modifications in the cell wall of lichen algae have also been described. Using the phycobiont of Xanthoria parietina as a study organism. The authors have shown that concanavalin A bound to the cell wall of lichen algae cultured in vitro, but not to the same algae in the symbiotic form (freshly isolated from the lichen). They have also isolated a crude protein fraction from the lichen, a component(s) of which bound to the cell wall of cultured, but not freshly isolated algae. Binding was correlated to cytochemical features present only on the cell wall of cultured algae. With the aid of a newly-developed solid-phase radioimmunoassay, the authors show that there are also detectable antigenic differences, as well as similarities, between freshly isolated and cultured phycobionts from X. parietina.