20 batches of leucocyte extract (16 from positive donors to PPD and Candida, one from a positive donor to PPD but negative to Candida and three from non-sensitive donors) were prepared by a modification of the method routinely used at our laboratory, in which cold saline was replaced by lymphocyte culture medium as the washing fluid for leucocyte suspensions, and in which the use for distilled water and lyophilisation was avoided during processing. Contrary to leucocyte extracts prepared by the routine method, the batches of transfer factor obtained from sensitive donors were capable of strongly and specifically enhancing blast transformation in cultures of normally unresponsive or weakly responsive leucocytes stimulated by specific antigens (Candida and PPD). A much weaker non-specific stimulatory activity on cultures not stimulated by antigen or stimulated by Con-A or PHA was also present in extracts obtained both from sensitive and non-sensitive donors.