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Five out of 18 primary explants of human carcinomata obtained at operation have been grown progressively for a minimum of one month in thymectomized, x-irradiated mice reconstituted with syngeneic bone marrow and subsequently treated also with anti-thymocyte serum. All the tumours which proliferated were of gastrointestinal origin, growing locally but not metastasizing although direct invasion of the ribs occurred in one case. No implanted breast carcinomata grew in this system.

FBJ osteosarcoma virus (FBJ-MSV) isolated originally from a spontaneously arising osteosarcoma in a CF1 mouse is the only known naturally occurring murine sarcoma virus (MSV). It is unique among strains of MSV in producing primarily sarcomata in mice. The capacity of tumour cells transformed in vivo by this agent to elicit specific transplantation immunity in syngeneic hosts was investigated. A low level of resistance (10(4)-10(5) cells) was consistently induced by implantation of x-irradiated (15,000 rad) tumours or surgical excision of developing subcutaneous grafts. By contrast intraperitoneal inoculation of virus containing cellfree extracts of FBJ-MSV sarcomata was a far less effective immunization procedure. Confirmatory evidence for the antigenicity of these neoplasms was obtained in tests in which preincubation of tumour cells with lymphoid cells from specifically immune donors inhibited in vivo outgrowth of the FBJ-MSV cells in untreated syngeneic recipients. The induction of host resistance to FBJ-MSV cells by immunization with identical and independently-induced FBJ-MSV tumours established that FBJ-MSV cells possess common cell surface antigenic specificities in a manner analogous to those of experimental neoplasms induced by other oncogenic DNA and RNA viruses. Since FBJ-MSV cells release infectious virus it was not possible in this system to establish whether the tumour-rejection antigen was cellular or virion in nature. The antigenic weakness of FBJ-MSV cells in syngeneic hosts is comparable with that of virus-induced murine leukaemias of the Gross (G) or "wild" type subgroup to which category FBJ-MSV also belongs. These features suggest that FBJ-MSV exemplifies naturally occurring sarcomagenic viruses more closely than those of the Friend-Moloney-Rauscher-Graffi (FMRGr) subgroup which in general induce highly antigenic neoplasms.

The primary immune response elicited by host antigens in a grafted population of immunologically competent cells has been compared in conditions where the same dose of parental cells were grafted simultaneously to F(1) hybrid embryos of 13 or 17 days of age. The enlarged chimeric spleens harvested 4 days later were analyzed for donor cell proliferation by using the sex chromosomes as karyological markers, and for proliferation of immunological activity by means of transfer to secondary hosts of the same genotype. Whereas the total number of dividing donor cells were on the average 16 times higher in 17-day than in 13-day hosts, the recovery of immunological reactivity showed a 6- to 7-fold difference in the opposite direction. The experiments cast doubt on the proposition that cellular proliferation is necessary for development of a primary immune response. They suggest that there exists an alternative way in which a primary immune response may unfold from involving a few to involving a much larger number of cells.