Labels based on noble metal nanoparticles and surface enhanced Raman scattering (SERS) opened new opportunities for the ultrasensitive detection of analytes. To date, however, SERS labels were mostly used for qualitative analysis, while leaving largely unexploited their potential for ultrasensitive quantitative assays. Here we synthesized SERS labels based on gold nanoparticles (AuNPs) obtained by laser ablation synthesis in solution and we developed a general method for the correlation of the SERS label concentration with the intensity of the Raman signal. We successfully used this method for the quantification of the number of AuNPs uptaken by PMA differentiated U937 macrophage cells. Our work shows that quantitative ultrasensitive assays by SERS labels are possible and points out some issues that must be considered when performing this type of analysis.