p21 is a small guanine nucleotide binding protein that is involved in intracellular signal transduction. Biochemical data suggest that the presence of the beta-phosphate is essential for strong binding of guanine nucleotides to the protein. Guanosine or GMP bind six orders of magnitude more weakly to p21 than GDP or GTP. Moreover, the thermal stability of the protein is dramatically reduced when bound to GMP or guanosine. We have crystallized C-terminally truncated forms of p21(H-ras), with guanosine or GMP bound, in the space groups P4(3)2(1)2, P2(1)2(1)2 and P2(1). The crystals diffract in the range 2.8-2.2 A. Details of the crystallization procedures, the characterization of the crystals and preliminary results of structure determination are described. An unexpected electron-density peak was found close to the position of the beta-phosphate in the phosphate-binding loop.