Antibodies to the OGFr-ARF peptide detected nuclear and cytoplasmic proteins in RPMI 8226 and U937. Peptide selected antisera to OGFr-ARF was used to probe nuclear and cytoplasmic (S100) extracts from myeloma and myelomonocytic cell lines. Arrows indicate proteins specifically detected by the antisera as judged by the ability of the immunizing peptide (OGFr-ARF), but not of an irrelevant tandem repeat peptide (MUC1), to block detection. The integrity of the extract was demonstrated by the presence of proteins in one of the two compartments, but not both. For example, while p54 was found in both nuclear and cytoplasmic fractions, p174 localized to the nuclear fraction only.