Anaplasma marginale was propagated in a tick cell line derived from Dermacentor variabilis embryos. The rickettsial organism was identified and monitored in culture by transmission electron microscopy and the indirect immunofluorescence technique, using specific monoclonal antibodies. Inoculation of the embryonic tick cell line with midguts of infected adult ticks (culture 1), nymphal ticks (culture 2) and adult ticks that were infected as nymphs and dissected as adults (culture 3) resulted in 3 continuous cultures of A marginale. Culture 1 had been maintained through 22 passages over a 11-month period; cultures 2 and 3 had been maintained for 18 passages over a 9-month period. Growth of A marginale in the cell line began in the area of the nuclear membrane at approximately 4 days after inoculation or transfer. Thereafter, the organisms were observed in inclusions scattered throughout the cytoplasm of the host cells. Maximal growth of the organism occurred at 7 to 14 days, after which numbers of inclusions rapidly decreased to minimal or undetectable levels. The organism began new cycles of growth with each 1:5 to 1:10 split and transfer of the host cells. Electron microscopy of recently infected cells revealed a morphology of the organism that closely resembled that observed in marginal bodies of infected erythrocytes. After several passages, A marginale organisms had a varied morphology and resembled the organism described in midgut cells of naturally infected ticks.(ABSTRACT TRUNCATED AT 250 WORDS)