The objective of the study was to examine the effects of trans-9-octadecenoic acid (elaidic) and trans-II-octadecenoic acid (trans-vaccenic) on uptake and alteration of exogenous fatty acids by mouse mammary epithelial cells. Cells from a subclone of the COMMA-D cell line were plated on uncoated plastic petri dishes and grown to confluence. Supplemental fatty acids bound to bovine serum albumin were added to the medium applied to the confluent cell cultures. Treatments included 200 JlM octadecanoic acid (CI8:0) as a control and 100 JlM CI8:0 with one of the following JlM ratios of cis-octadecenoic acid (cis-CI8:I) to elaidic or trans-vaccenic: 100:0, 75:25, 50:50, 0:100. In addition, all treatments were conducted with or without lactogenic hormone supplementation. The cellular protein to DNA ratio and total amount of fatty acids per mg protein were decreased (P < .05) by addition of lactogenic hormones. In treatments without hormone supplementation, however, the total amount of cellular fatty acids per mg protein was decreased (P < .05) by addition of either trans-CI8:I isomer. Results indicated a significant (P < .05) relationship between the concentration oftrans-C18:1 in the media and uptake of trans-C 18: 1 isomers, and retroconversion of trans-C 18: 1 to trans-C 16: 1. The slopes of the lines for cellular C16:0, cis-C16:1, and cis-CI8:1 were less (P < .05) than zero as concentration oftrans-CI8:1 in the media increased. However, trans-CI8:1 isomers did not influence the proportion of polar and nonpolar lipids synthesized by the cells. It appears that trans fatty acids may depress milk fat output by decreasing de novo fatty acid synthesis and cis-C 18: I content. / Master of Science