Specific antibodies against rap1A and rap1B small GTP‐binding proteins were generated by immunization of rabbits with peptides derived from the C‐terminus of the processed proteins. Immunoblot analysis of membranes from several mammalian cell lines and human thrombocytes with affinity‐purified antibodies against rap1A or rap1B demonstrated the presence of multiple immunoreactive proteins in the 22–23 kDa range, although at strongly varying levels. Whereas both proteins were present in substantial amounts in membranes from myelocytic HL‐60, K‐562 and HEL cells, they were hardly detectable in membranes from lymphoma U‐937 and S49.1 cyc⁻ cells. Membranes from human thrombocytes and 3T3‐Swiss Albino fibroblasts showed strong rap1B immunoreactivity, whereas rap1A protein was present in much lower amounts. In the cytosol of HL‐60 cells, only small amounts of rap1A and rap1B proteins were detected, unless the cells were treated with lovastatin, an inhibitor of hydroxymethylglutaryl‐coenzyme A reductase, suggesting that both proteins are isoprenylated. By comparison with recombinant proteins, the ratio of rap1A/ras proteins in membranes from HL‐60 cells was estimated to be about 4:1. An antiserum directed against the C‐terminus of rap2 reacted strongly with recombinant rap2, but not with membranes from tested mammalian cells. In conclusion, rap1A and rap1B proteins are distributed differentially among membranes from various mammalian cell types and are isoprenylated in HL‐60 cells.