SUMMARYHuman cells derived from both normal and neoplastic tissues can be infected by Mason-Pfizer monkey virus (MPMV) without accompanying cytopathology. Infection of cell cultures such as human rhabdomyosarcoma (A2o4) results in a persistently infected cell line which can be subcultured over 3o sequential culture passages without significant change in phenotypic properties according to reverse, transcriptase (RT), MPMV p27 antigen content, virus particle count and infectivity titrel Productive virus infections were established at relatively low virus particle (VP) input multiplicities (p.i.m. ; about o.o6 VP/cell) in A2o4 cell cultures. At higher p.i.m. (about 6oo to 6ooo VP/cell) newly synthesized virus was detected within 4 days post infection. Although virus production was cumulative following primary infection, after subculture of infected cultures MPMV production was greater during active cell division. Using synchronization techniques, MPMV replication in persistently infected cultures was found to be cell cycle-dependent. The major internal antigen, P27, was synthesized in G2 and newly synthesized virus particles were released predominantly during mitosis and early G1. Colcemid arrest of cells during mitosis inhibited subsequent MPMV release. Consequently, production of extracellular virus depends upon the progression of cells through the mitotic stage. These data, which provided a basic understanding of the virushost relationship that occurs in primate cells productively infected with MPMV, were used as a guideline for isolating MPMV-like viruses from experimentally and naturally infected Rhesus monkeys.

Summary. The normal human Iymphoc>t(' population which exhibits "spontaneous" cytolysis of EH^ Burkitt's lyiiiphuma cells has been cliaracterized. The effector cell has E.\ and EAC receptors but lacks E receptors and probably .surface Ig. "Spontaneous" anti-HB^ c> totoxieilv was not reduced by preiiicubation of the L-dector cells with plastic (»r Iron carbonyl or by pa,s.sage through cotton WCKJI or aj;an)se ciiluinns hut WILS reduced by pa.s.sage through nylon wool coiumn.s. Thy mot)-les were not cytotoxic to EB