Building on early gains, the sensex at the Bombay Stock Exchange had advanced a good 173 points, or 0.9 per cent, to 19,428 in afternoon deals, a short while ago. Earlier in the morning, the Sensex had opened with a more moderate gain of 66 points, or 0.3 per cent, at 19,321, on renewed buying by speculators, ahead of tomorrow's monthly expiry in the derivatives segment.