When Moloney lymphoma (YAC) cells were freshly explanted from the tumor bearing host into culture, 2 events occurred in the first 120 min: the cells lost their natural IgG coat, and their sensitivity to complement dependent lysis (CdL) mediated by antibodies to Moloney lymphoma cells decreased or increased. An increasing sensitivity to CdL as a function of incubation time at 37°C was likely to occur when the sensitivity to CdL was low at explantation. A decreasing sensitivity to CdL was probable in instances of a high sensitivity to CdL at explantation. Artificial coating of YAC cells with antibodies to Moloney lymphoma immediately after explantation moderated the alterations in their sensitivity to CdL. This occurred even though a functional antibody did not remain on the cells as evidenced from the gradual decreased sensitivity of these artificially coated cells to the addition of complement. Spent culture media in which freshly explanted cells grew for 60 or 120 min sometimes blocked CdL of YAC cells mediated by antibodies to Moloney lymphoma.