The following characteristics of the effector cells for natural cytotoxicity (NC) against BALB/c Meth A tumor cells in BALB/c and other mouse strains were observed: 1) time course studies showed that most of NC was completed by 12 to 24 hr of incubation; 2) pre-incubation at 37°C for as long as 6 days had no effect on NC activity; 3) pre-treatment with ammonium chloride had no effect on NC activity; 4) NC cells were not adherent to plastic surfaces; 5) NC cells showed some degree of adherence to nylon wool fibers and activity was recovered from the adherent population; 6) NC cells were partially retained by G10-Sephadex columns; 7) by using velocity sedimentation at unit gravity in Ficoll gradients, NC cells were homogeneous having an average sedimentation velocity of 4.3 to 4.5 mm/hr; 8) treatment with carbonyl iron and magnetism had no effect on NC activity; 9) treatment with various antisera + C including, anti-Thy 1, anti-Lyt 1, anti-Lyt 2, anti-Ala 1, anti-NK, rabbit anti-mouse brain, and rabbit anti-mouse Ig, had no effect on NC activity; 10) heat-aggregated Ig had no effect on NC-mediated CMC; 11) trypsin treatment decreased NC activity but the effect was reversible, dependent on trypsin dose, and detected only in short-term CMC assay. These properties were similar for NC cells obtained from normal or nude mice, as well as peritoneal exudates obtained 5 days after i.p. BCG. When compared to the properties of the NK cell, many similarities were evident although differences in sensitivity to pre-incubation at 37°C and in the relative degree of adherence to nylon fibers were found. It is proposed that both the NC and NK cells belong to a family of effector cells with similar functions.