One has attempted to observe the relation which exists between peritoneal histiocytes and Brucella melitensis after uptake of the latter by normal and immune cells in vitro. To this end maintenance and growth of the histiocytes are essential for relatively long periods in order to be certain of the vitality of the host cell, although the end-point of the experiment is available within 3-4 days of incubation. Indeed the major outlines of the response of histiocytes to the cytotoxicity of the brucellae are manifest within the first 24-hour period of incubation, after which further degeneration or continued survival followed by multiplication is principally a function of the relative virulence of the brucellae.The data will show that peritoneal histiocytes from rabbit, monkey and guinea pig do not differ in their cytotoxic response. This response is non-specific in terms of the species of brucella being studied for immunization and challenge infection.Methods. Histiocytes were harvested and treated as previously described by Elberg et al( 1 ) . The cells were exposed to B. melitensis, strain Rev I, which had been grown on Albimi agar and washed 3 times in modified Tyrode so1ution.t A ratio of 10 bacteria: 1 monocyte was used for parasitization. The cells were then cultivated in either Mackaness or Sykes-Moore microchambers. Normal cells were cultivated in Tyrode + 40% normal *Supported by grants from Nat. Inst. Health and Nat. Science Foundation. rabbit serum; immune cells were grown in 40% immune serum in Tyrode solution. Samples were taken at 3, 24, 72, 120, and 168 hours.Results. Maintenance was well demonstrated in the chamber cultures. The customary degeneration after parasitization of histiocytes, derived from normal animals, and due to the cytotoxic action of the brucellae, is seen in the data of Table I which presents a sample from many animals and chambers giving identical results.A comparison of Hartley strain of guinea pig and Cynomolgus phillipinensis histiocytes TABLE I. Survival and Multiplication of Rabbit Peritoneal Histiocytes Following Parasitization by B . melitensis, Strain Rev I." Ohr 24hr 48hr 9 6 h r ~~ Non-parasitized (a) normal cells (b) Parasitized (a) normal cells (b) Non-parasitized (a) immune cells (b) Parasitized (a) immune cells (b) Attenuated vaccine strain. t Modified Tyrode Solution: Solution A : NaCcI, 7.7 g ; KCl, .2 g ; MgSO, (anhydrous) .1 g ; Dextrose, 4.0 g ; NaH2P0,, .05 g ; Water, 750 ml. Solution B : NaHCO,, .5 g ; Water, 25 ml. Double-distilled water is used. The solutions are autoclaved separately and combined when cool. If an indicator is desired, .1 ml of phenol red (La Motte Solution, 1%) is added to Solution A before autoclaving.