The homologous proteins Oxa1, YidC and Alb3 mediate the insertion of membrane proteins in mitochondria, bacteria and chloroplast thylakoids, respectively. Depletion of YidC in Escherichia coli affects the integration of every membrane protein studied, and Alb3 has previously been shown to be required for the insertion of a signal recognition particle (SRP)-dependent protein, Lhcb1, in thylakoids. In this study we have analyzed the 'global' role of Alb3 in the insertion of thylakoid membrane proteins. We show that insertion of two chlorophyll-binding proteins, Lhcb4.1 and Lhcb5, is almost totally blocked by preincubation of thylakoids with anti-Alb3 antibodies, indicating a requirement for Alb3 in the insertion pathway. Insertion of the related PsbS protein, on the other hand, is unaffected by Alb3 antibodies, and insertion of a group of SRP-independent, signal peptide-bearing proteins, PsbX, PsbW and PsbY is likewise completely unaffected. Proteinase K is furthermore able to completely degrade Alb3 but this treatment does not affect the insertion of these proteins. Among the thylakoid proteins studied here, Alb3 requirement correlates strictly with a requirement for stromal factors and nucleoside triphosphates. However, the majority of proteins tested do not require Alb3 or any other known form of translocation apparatus.