The invention relates to a method of preparing a radiolabeled technetium complex by bringing together a pertechnetate solution in a complex-forming reaction with a compound of general formula (I) wherein R1-R12 independently represent H, C1-C4 alkyl, a carboxy group, or carboxy (C1-C4) alkyl; or wherein R1 + R2, R3 + R4, R5 + R6, R7 + R8, R9 + R10 and R11 + R12 independently represent an oxo or imino function; and wherein: R13 is H, C1-C4 alkyl or carboxy (C1-C4) alkyl; R14 is H or NH2; n is 0, 1 or 2; A represents NH2 or SH; and B represents SH, or a group of the general formula NH-(Z-CO-NH)-mZ-COOH; said reaction performed in the presence of Sn(II) as a reducing agent, in the absence of a transfer agent, in a substantially aqueous solvent system having a pH of at least 10, and at ambient temperature. The invention further relates to a kit for performing said method.