Dipel (Abbott Laboratories, N. Chicago, U.S.A.) and Bacilan (PPR, Wałcz, Poland), commercial preparation of Bacillus thuringiensis var. kurstaki, containing 16,000 IU/mg were tested in vitro on Menopon gallinae (L.) and Eomenacanthus stramineus (Nitzsch). The insects were placed in feathers, in Petri plates, and sprayed with 0.25, 0.5, 0.75, 1, 2, 4 and 10% solutions of insecticides. Each concentration was tested on a group of 10 insects, in three replications. The viability was observed after 10, 20, 30, 60 min and then 10 times within 1 hr intervals. Dipel was effective in lower concentrations [LC50 = 2.2 (1.4-3.5)% against E. stramineus and 2.9 (1.8-4.6) against M. gallinae]. LC50 of Bacilan were 2.9 (1.8-4.3) for E. stramineus and 3.2 (2.5-4.0)% for M. gallinae respectively. The ingestion of microbial insecticides was proved by the presence of spores B. thuringiensis in the insects being previously exposed to indirect activity of 2.2% Dipel and 2.9% Bacilan. Moreover, the growth of B. thuringiensis colonies in the lice culture was observed. 3% Dipel solution proved to be effective in killing the insects on the host, but no earlier than after the second application within 24 hr intervals.