heterogeneity in the sensitivities of the 13762nf rat mammary adenocarcinoma cell clones to cytolysis mediated by extra and intratumoral macrophages the susceptibility of cloned cell lines of the 13762nf rat mammary adenocarcinoma to macrophagemediated cytolysis was investigated using both intra and extratumoral macrophages the percentage of fc receptorpositive cells in tumors growing sc in syngeneic f344 rats ranged from 8 to 20 but we could not demonstrate a significant correlation between the number of fc receptorpositive cells within tumors and their spontaneous metastatic potentials in macrophagemediated cytolysis assays cloned 13762nf cell lines of differing metastatic potential established from tissue culture lines fresh tumor explants or shortterm cultures one passage in vitro were used as targets effector cells were thioglycolateelicited peritoneal macrophages activated in vitro with bacterial lipopolysaccharide or intratumoral macrophages activated in vitro with lipopolysaccharide when the effector cells were peritoneal macrophages established cloned 13762nf cell lines showed little correlation in their susceptibility to macrophagemediated cytolysis and metastatic potential while this was not observed when fresh tumor explants were used highly metastatic mtln3 cells were the least sensitive less metastatic mtf7 and mtln2 cells were more susceptible and the low metastatic parental mtpa cells were the most sensitive in 72h cytolysis assays when the effector cells were intratumoral macrophages all 13762nf cell lines showed less sensitivity in cytolysis assays than similar assays using thioglycolateelicited peritoneal macrophages with the exception of line mtln2 shortterm cultures one passage in vitro did not differ substantially in susceptibility to intratumoral macrophages compared to fresh explants in this system the sensitivity of 13762nf cells to macrophagemediated cytolysis is a function of effector as well as target cell source