This study was performed to determine what factors related to the enzymatic isolation technique and assay conditions may influence the measurement of spontaneous cell-mediated cytotoxicity (SCMC) of mononuclear cells (MNC) isolated from human intestinal mucosa. In 18 studies, the SCMC of freshly isolated cells was 1.8 +/- 0.4% but increased to 12 +/- 3% following 24 hr culture without a change in the proportion of cells with the NK phenotype (Leu-7+). The SCMC tended to plateau with more prolonged culture. Culturing peripheral blood (PB) MNC for 24 hr did not alter SCMC nor the proportion of Leu-7+ cells. However, the suppression of the SCMC of PBMNC preincubated with the supernatant of the collagenase digestion of intestinal mucosa was completely reversed by 24 hr culture. Intestinal MNC were found to suppress the SCMC of autologous PBMNC in mixing experiments. However, 24 hr culture did not affect the degree of suppression and the proportion of T cells of the suppressor-cytotoxic phenotype (Leu-2a+) was also unchanged. It is concluded that the SCMC of intestinal MNC may be accurately assessed following 24 hr culture of the cells to allow recovery from the suppressive influences of the isolation process and that this does not introduce other artefactual problems. However, suppression of cytotoxicity within the assay may result in an underestimation of the SCMC of intestinal MNC when compared to that of PBMNC.