Extensive protease digestion of delipidated [H-3]mevalonate (MVA)-labeled proteins, followed by HPLC separation of the products, is one approach to identify and study prenyl cysteines. Using this methodology three major [H-3]MVA-labeled peaks appeared. Two of them represent farnesyl cysteine (FC) and geranylgeranyl cysteine (GGC). The third peak represents unknown products that are considerably more hydrophobic than FC and GGC, here designated HPC. Previously, we provided evidence that cysteine residues may also be modified by dolichyl groups. Dolichyl cysteines (DoIC) belong to HPC. However, as shown in the present study, DoIC only represents a minor portion of HPC. Data obtained from different sets of experiments, including [H-3]GGOH-labeling and use of prenyl transferase inhibitors, suggest that HPC mainly involves CXC or CC residues with double-linked GG groups. In turn this points to the possibility that proteins modified by double GG groups are quite common, and may probably involve other proteins than the rab family of GTPases.