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Immune responses are the result of extended clonal growth and maturation of specific precursors preexisting in low frequencies. Clonal expansion is partly controlled by soluble factors, thus, their identification is essential for the understanding of the immune response regulation. I describe a T-T hybridoma (TUH-3) produced B cell growth factor (BCGF or B cell stimulating factor, BSF), distinct from T cell growth factor, supporting the growth of lipopolysaccharide (LPS)-preactivated B cell blasts. BCGF was unspecific with respect to Ag, MHC or Ig haplotypes and was poor in supporting the development of Ig-secreting plaque-forming cells (PFC) in B cell blast cultures. The producer hybrid was unstable. Later produced BCGF/BSF-secreting hybrids were shown to be contaminated with Mycoplasma arginini and the BCGF/BSF cosedimented with the microorganism. The M. arginini BSF-like activity fulfilled all of the above mentioned criteria of a lymphokine. Thus, a host-parasite relationship with M. arginini mimicking the effects of a BSF on B cells was recognized. In the thesis I adress questions concerning the identity of the hybridoma factor, the existence of growth factors for LPS-preactivated B cells and design experiments to define molecules on mycoplasma with lymphokine-like activity. Comparison of in vivo effects showed that M. arginini or TUH-3 conditioned medium (CM) differed in parameters as cell growth in bone marrow and spleen and antibody isotype expression. The TUH-3 factor is thus concluded to be distinct from the effect of the mycoplasma contamination. In studies of CM from concanavalin A stimulated murine spleen cells a factor supporting the growth of LPS-preactivated murine spleen B cells was found as a highly acidic B cell growth stimulatory activity. It was distinct from other known factors and interleukins (IL) based on its biochemical properties. Furthermore, it was shown that none of the recombinant DLs, alone or in combination, were able to support the growth of LPS-preactivated B cells. Thus, we conclude that we have identified a growth factor specific for LPS-preactivated B cells. Mycoplasmas are a heterogeneous group of prokaryots causing a wide variety of diseases, amongst others, reumatical and autoimmune disorders. Since M. arginini induced a lymphokine-like effect it was hypothesized that this was due to molecular mimicry of a lymphokine. We have been able to isolate four membrane bound acyl proteins, which support the growth of LPS-preactivated B cells. These proteins were shown to be distinct by peptide mapping and to have lipids attached. Since there are several other acyl proteins, which do not support the growth of LPS-preactivated B cells, it is concluded that these proteins show a previously undescribed specificity. digitalisering@umu