A fibroblast-like cell line derived from the mountain vole Microtus montanus was effective in promoting an increase in the multiplication rate of new and established isolates of Plasmodium falciparum. An increase in multiplication rate was also achieved in the absence of M. montanus cells when cultures of P. falciparum were grown in medium which was supplemented with L-cysteine at 12 h intervals, and to a lesser extent when medium was supplemented with 2-mercaptoethanol instead of L-cysteine.