In vitro cultivation of murine spleen cells resulted in a spontaneous release of receptors for alloantigens. This was revealed by the capacity of cell-free culture supernates to recognize alloantigens as measured in the PAR assay. Qualitatively, recognition responses obtained with these supernates reproduced faithfully those found with the corresponding cells. Large amounts of receptors were released by untreated spleen cells and by spleen cells treated with a rabbit antimouse immunoglobulin serum and complement, smaller amounts were released by bone marrow cells, and native thymus cells released none. Spleen cells from nude mice and spleen cells from normal mice treated with anti-theta serum and complement showed no release of receptors. It was concluded that receptors active in the PAR test were of T-cell origin. Release of T-cell receptors was found to be discontinuous and proceeded in waves. The amount of released receptors depended on the number of cells cultivated. Release occurred at 37 degrees C but not at 4 degrees C. Interaction with antigen, however, was temperature-independent. In contrast to T-cell receptors, a release of H-2 antigens could not be detected with the culture conditions employed.