By means of a formula, developed by J. Schubert and A. Catsch, H.J. Heller, as well as a relation postulated by A. Catsch, the 'thermodynamic' stability constant of the Radium (222)-cryptate K(Ra)(Ra)(222) was calculated from measurements of the total body retention of the radioactive alkaline earthes 85SR, 140Ba and 224Ra and its (222)-cryptates in rats. From the same in vivo data, a direct lineary relationship between the log of the effectiveness quotient, log EQ(M(222)), and the log of the 'thermodynamic' stability log K(M)(M)(222) was found graphically. The values from the graph correspond with those of the calculation.