When in vitro growth of Vittaforma comeae was tested using MDCK, MRC-5, XEN, L-929 and FHM cell lines, propagation occurred only in MDCK, MRC-5 and XEN cells. The intervals required for the various stages of the life cycle to develop were the same in all the cell lines tested. The MDCK cell line was selected to support the growth of V. comeae in vitro and provide the system for in vitro testing of drugs. The weekly output of V. comeae spores from the MDCK cell monolayer was monitored over a 61-week period during which there were fluctuations but no definite increase or decrease in output. Albendazole at 2.1 or 4.2 pg/ml in MEM was tested against V. comeae in MDCK cell monolayers and showed antimicrosporidial activity. The percentage of infected cells was reduced in the presence of the drug and there were ultrastmctural abnormalities in all stages of the life cycle. The drug prevents parasite division.