Studies of the rates of release of IgM were performed in vitro and in vivo. Short term in vivo studies in which 4.98 x 106 tumor cells/mouse were given i.v. in order to quantitate cellular synthetic function showed that the synthetic rate decreased progressively. Samples taken within 1 hr of injection gave rates of release similar to those in the in vitro studies. Nine in vivo samples taken at 30 min gave a rate of 74.03 ± 18.37 and 13 in vivo samples taken at 1 hr gave 89.98 ± 9.97 molecules/cell/sec. Seventeen separate in vitro measurements gave 76.30 ± 7.44 molecules/cell/sec. The low rate of release of MOPC tumor IgM as compared to the primary response IgM hemolysin to sheep red blood cells is discussed.