The effect of a cell suspension of Propionibacterium granulosum strain KP-45 given intravenously to patients with advanced primary or secondary lung carcinoma, and simultaneously treated with cytostatic drugs, was estimated. The control group consisted of patients treated with cytostatics only. The clinical state of patients treated with the KP-45 preparation and with this immunostimulant plus cytostatics, was compared by evaluation of tumour size by X-ray examination, peripheral blood picture, and biochemical and immunological parameters. Treatment with P. granulosum KP-45 did not cause any serious adverse reaction or complication. No evident inhibitory effect on primary or secondary lung tumour development was observed. However, injections of the KP-45 preparation in cytostatics-treated patients resulted in decreased adverse effect of these drugs on the haemopoietic system and absence of bacterial infections, in contrast to patients treated with cytostatics only.