Polyclonal antibodies were raised against the purified ribosomal proteins L1 and L2, the 5S rRNA binding protein L3, all from Saccharomyces cerevisiae, and against L1 and L2 from Schizosaccharomyces pombe (numbering according to Otaka and Osawa 1981; Otaka et al. 1983, respectively). For clarity prefixes Sc and Sp have been added to the numbering of proteins derived from S. cerevisiae and S. pombe, respectively. Ribosomal proteins from these yeasts and from Kluyveromyces marxianus, Rhodotorula glutinis, the slime mold Dictyostelium discoideum and the protozoan Tetrahymena thermophila were checked for antigenic cross-reactivity by the immunoblot technique. Anti-ScL1 bound to the largest ribosomal proteins of all organisms but not with equal strength. A fast migrating protein band from R. glutinis was also reactive. Anti-ScL2 reacted strongly with L2 or analogous proteins derived exclusively from the yeasts. Anti-ScL3 cross-reacted only with one protein band from K. marxianus, whereas anti-SpL1 cross-reacted with L1 or its analogues from the other organisms, but also with proteins of lower molecular weight. In S. cerevisiae, these proteins are located exclusively on the small ribosomal subunit. L2 or analogous ribosomal proteins of all organisms were recognized by anti-SpL2 but additionally the ribosomal protein YL28 of S. cerevisiae and fast migrating proteins of T. thermophila exhibited anti-SpL2 binding.