Cultures of primary bone marrow cells, obtained from the tibiae and femora of hamsters (HBM), mice (MBM), or rats (RBM) were inoculated with red cells infected with Plasmodium berghei, P. vinckei vinckei, or P. knowlesi. Merozoites, rings, trophozoites, schizonts and a few gametocytes were seen in HBM and MBM cells inoculated with P. berghe-infected red cells. At 1 to 2 or 3 days after inoculation in HBM and MBM, the number of intracellular asexual stages decreased slightly or remained the same, whereas at 3 to 7 or 8 days a 4 to 7-fold increase occurred. Parasites then decreased in number from days 8 to 11 and no parasites were seen on day 12. Intracellular parasites were most numerous at 7 and 8 days in MBM and HBM cultures, respectively. Mice injected with supernatant or cells from control cultures (without cultured cells) or primary cultures inoculated 1 and 2 days earlier with P. berghei- or P. vinckei vinckei-infected red cells always became infected. Mice injected with material from 3-day-old control and experimental cultures usually became infected, and those injected with 4- to 12-day-old cultures never became infected.