A standardless X-ray microanalytical procedure has been developed to determine the number of gold-labelled surface receptors on whole single cells. The effect of the injection of K2PtCl4 into mice on gold-labelled concanavalin A (Con A) receptors on peritoneal macrophages was examined with an energy dispersive X-ray detector in an SEM. The numbers of gold particles seen in electron micrographs and estimated by fluorescence photometric measurements of fluorescein isothiocyanate-labelled Con A receptors were correlated with the X-ray microanalytical results.