I suspect that it depends on two things:
(1) whether you generally have the y sound before the letter “u”, e.g. do you pronounce “tune” as “tyoon” or “toon”? (The latter is general in the US, the former is used in the US and other countries).
(2) whether you want to get close to how Latin was pronounced 2,000 years ago, or just want to pronounce Latin words like English words. The Ancient Romans would have pronounced the “i” in “situ” like the “i” in “sit”, not like the “i” in “site”.
I had no idea there were so many variations on this pronunciation.
I’ve always pronounced it, and heard it pronounced, “in see-two,” like trupa, mischievous, and vetbridge. I hadn’t thought to pronounce it “SIGH too” or “si-choo” until reading these responses, and looking up the dictionary guides.
Each dictionary seems to have a different preference, too, although most list all three of these (and “si-tyoo” or other variants).
I wonder if these variations are related to different disciplines? I note that the term is frequently used in medicine and biology, as well as in archaeology and art history. I don’t know if there’s a pattern here or not (I’m in art history, for what it’s worth), but a poll might prove interesting.
Eh, adam, pronunciation must truly be a meme at some level. Like why so many doctors pronounce centimeter “sontimeter”. I didn’t mean my comment in a stuck-up-we’re-right kind of way, I meant it in a only-an-observation kind of way. I know better than to claim that biologists have some sort of natural gift with pronunciation (we are pretty miserable at it – look at what we do to the word apoptosis). Just saying that iif the OP wanted to know because they were in the process of using Sp6 or T3 polymerase to synthesize a digoxygenin-conjugated riboprobe for hybridization to whole mount samples followed with detection by a alkaline phosphatase conjugated antibody and subsequent color reaction with NBT/BCIP to illuminate transcriptional control of a gene, they would want to know how everyone else is going to say it.
It actually came up because it was one of those words that I’ve read but never said, and was unsure of the pronunciation. To avert future embarassment, I looked it up, but was quite annoyed when I didn’t get an authoritative answer. So all the answers here are useful.