Indirect fluorescent antibody staining was used to examine the replication of lactate dehydrogenase-elevating virus (LDV) in primary cultures of peritoneal macrophages from BALB/c mice of different ages. Up to 80% of the total peritoneal macrophages from 1-2-week-old mice were susceptible to productive infection by LDV, though only 1-2% of the cells expressed detectable levels of IA antigen. The proportion of LDV-permissive peritoneal macrophages progressively decreased to 5-15% between 2 and 5 weeks of age of the mice. Macrophages from 9-day-old mice, when cultured in the presence of L cell conditioned medium, retained undiminished LDV permissiveness for at least 10 days in culture. The maximum proportion of LDV antigen-positive cells was detected between 8-10 h post infection of macrophages cultured from both 1-2-week-old and adult mice, concomitant with maximum LDV RNA synthesis. The LDV antigen positive macrophages disappeared between 12 and 48 h post infection. In cultures of macrophages from 9-10-day-old mice, the loss of infected cells was clearly due to cell killing, proving unequivocally that LDV replication is cytocidal. Disintegration of LDV-infected macrophages or phagocytosis of killed macrophages by surviving macrophages must be very sudden and complete since infected cells disappeared without the appearance of trypan blue-stainable cells in the culture. Ten cell lines established from macrophages of 2, 9, and 10-day-old mice all contained a small proportion of LDV-permissive cells (1-4%). Individual clones of one of the lines contained a similar small proportion of LDV-permissive cells.