We have found that in 15 of 15 primary human colon tumors there was a significant decrease (by about 40%) in the levels of diacylglycerol when compared to paired adjacent normal mucosa samples. Assays on the same samples indicated that this decrease was seen both in tumors that did and did not display mutations in codon 12 of c-K-ras. These results, taken together with previous studies on protein kinase C, suggest that the protein kinase C signal transduction pathway is suppressed in human colon cancer.