The primary objective of this study was to determine if a small amount (0.5%) of deodorized menhaden oil (DMO) Significantly promotes or accelerates the oxidation of soybean oil (SO). A blend (M/S) of 0.5% DMO in commercial SO, pure SO, DMO and undeodorized menhaden oil (UDMO) were stored in the dark for ten weeks at 30°C. A control or "on the spot" blend was obtained by adding 0.5% DMO to SO prior to testing. The peroxide, anisidine and Rancimat values were determined at weeks 0, 1, 2, 3, 4, 6, 8 and 10. Fifteen panelists determined the differences in rancidity of the control and M/S using the directional paired-comparison test. The peroxide values of the control peaked at weeks 2 and 6 while those of M/S reached maxima at weeks 2 and 8. M/S was slightly but not significantly more stable to oxidation than the control. The peroxide values of UDMO reached a maximum at week 2, earlier than those of DMO. The anisidine values of the two blends were below 2.00 throughout the study. Although the anisidine values of M/S fluctuated more than the control, they were not significantly different from the control over the ten week study. The anisidine values of DMO were always less than those of UDMO. The Rancimat values of M/S were significantly (p<0.05) greater than the control at weeks 2, 6, and 10, and insignificantly greater at weeks 0, 1, 3, and 8. M/S was determined to be significantly (p<0.05) less stable than the control only at week 4. M/S was judged to be significantly (p<0.05) more rancid than the control only at week 3. Most of the results of this study do not support the hydroperoxide seeding action hypothesis of Josephson et al. (1984). / Master of Science