Thymocytes from juevenile Xenopus laevis did not proliferate in response to commercial preparations of lipopolysaccharide (LPS), responded poorly when cultured with the T-cell mitogen, phytohaemagglutinin-P (PHA), and were not co-stimulated by PHA plus LPS. However, supernatants (SNs) from LPS-treated cultures of adult Xenopus macrophage-enriched resident peritoneal cells (PCs) enhanced the proliferative responses of thymocytes to a submitogenic dose of PHA. These SNs were incapable of supporting long-term growth of thymic lymphoblast cell lines, and thus could be distinguished from T-cell growth factor (TCGF)-rich SNs, which were essential for propagating these cells. The co-stimulatory activity was present in 0-24-hr SNs; after 48 hr, SN activity declined. No functional cross-reactivity of mammalian and Xenopus interleukin-1 (IL-1)-rich SNs was detected. These data are consistent with the proposition that a macrophage-derived factor, functionally homologous with mammalian IL-1, can enhance a T-cell proliferative response in an amphibian.