We have investigated the in vitro transformation of lymphocytes that have been frozen and stored at low temperatures. Cells frozen at different times and stored for various times do transform in a reproducible manner when placed in culture with PHA or as one population of the two-way or the one-way MLR. When frozen-stored lymphocytes are cultured with specific antigen or as both partners in the MLR the response is minimal. The freezing process destroys neutrophils, and the remaining population transforms in a manner similar to cultures of purified lymphocytes. Practical aspects of the technique are discussed and we have suggested possible practical applications for future investigation.