The influence was studied of mass B. trigonocephalum infection on the isoenzyme spectrum of serum malic dehydrogenase in adult sheep and lambs. The investigations were conducted prior to infection, on day 10 post infection and following occurrence of clinical picture of bunostomiasis. It was found that on day 10 post infection, the serum of experiment sheep and lambs contained new fractions of malic dehydrogenase, NAD-specific, NADF-specific. They were observed also post occurrence of bunostomiasis clinical symptoms. The new fractions possess different electrophoretic mobility from that of malic dehydrogenase isoenzymes observed in the serum of experiment animals prior to infection. Their percentage ratio with respect to the remaining isoenzymes increases with disease development. Change in the percentage ratio of malic dehydrogenase isoenzymes normally found in sheep and lamb serum observed post infection.