Boar spermatozoa revealed three prominent resonances in the 3 1 P-NMR spectrum of intact cells. Two of these are known to be GPC and P i , the third is a phosphomonoester (PME), the identification of which was carried out by proton-detected 2D 1 H, 3 1 P and 1 H, 1 3 C chemical shift correlation experiments with gradient selection. The PME was unambiguously assigned to adenosine 5 -monophosphate (AMP). The identification was confirmed by an AMP consuming enzymatic assay. Other physiologically relevant PME's, in particular inosine 5 -monophosphate (IMP) and sugar phosphates, were excluded. The intensity of the 3 1 P signal of AMP in boar sperm extract was much higher than those of ADP and ATP, and in intact cells only AMP but no ATP was visible.