In the fungus Podospora anserina, two mutants with ribosomes apparently lacking one ribosomal protein were investigated using immunological techniques to determine whether these proteins were indeed missing. Antibodies directed against the wild type ribosomal proteins apparently absent in the mutants were raised from two-dimensional gel spots containing these proteins. These sera were used to detect cross-reacting proteins in the mutants. Immunological results have revealed in both mutants the presence of strongly altered ribosomal proteins which co-migrate with other wild type proteins on two-dimensional gel. The molecular weight of mutant proteins differs at least 25% from that of homologous wild type proteins. Furthermore, we observed that the growth of the mutants and the activity of their ribosomes are very reduced in both single mutants containing the altered proteins.