The application of a mathematical model was tested by a murine myeloma model. The plasmacytoma MOPC 104E secretes an IgM antibody to bacterial dextran B 1355. By using this system the various parameters required in the equation could be quantitated. The model relates the number of tumor cells in the population to antibody produced under conditions of logarithmic growth. Experimental values of antibody measured and estimation of starting population (An) of tumor cells showed that 2.24 x 106 cells grew when 3 x 106 cells were injected. This indicates that approximately 75% of the cells grew when 3.0 x 106 tumor cells were given i.p. into each mouse. The experimentally determined antibody levels were in agreement with the computer calculated values.