Abstract A method of direct freezing was found to be reliable for storage of the effectors for the LDA assay, though there is a considerable loss of cells. In order to get immunologically active K cells after thawing, an important step is to leave the cells to recuperate overnight in an appropriate medium before use. A short-term storage of fresh effectors is also possible. The use of a pool of effector cells from a minimum of 3 random blood donors obviates the selection for HL-A types furthermore permits a saving of the amount of serum used. Enrichment of a B cell population by removing the cells capable of forming spontaneous rosettes with SRBC was not found to enhance cytotoxicity. Pretreatment of cells with pronase, trypsin and neuraminidase did not significantly enhance their effector activity, while papain treated cells showed a systematically increased specific lysis.