Keith Fahey's goal just before half-time handed promotion-chasing Birmingham victory against struggling Southampton at St Andrew's.
Fahey scored what proved to be the winner when he drilled in Cameron Jerome's clever ball from the left.
Blues also went close when striker Scott Sinclair hit the post in the closing stages.
Southampton rarely tested the Birmingham goal, although Jason Euell had his lob cleared off the line.
Birmingham City manager Alex McLeish: "The last four days have been excellent. The players have responded to the challenge.
"We have wasted some good opportunities but we are still in a strong challenging position and I am proud of them. The back five were rock solid.
"No-one should forget we have lost key players for long parts of the season and we have been hard done by."
Southampton manager Mark Wotte:
"What concerned me was conceding a goal just before half-time.
"We cannot concede a goal like that after they had played for 10 minutes with only 10 men when Lee Carsley was off the field.
"Birmingham are a strong side but we were not strong enough up front to score a goal. Now I have to lift the side for the two home games against QPR and Derby."
Birmingham: Maik Taylor, Carr, Jaidi, Martin Taylor, Queudrue, Larsson, Carsley, Bowyer, Fahey (Sinclair 68), Jerome (Bouazza 76), Bent (Costly 18).
Subs Not Used: Doyle, De la Cruz.
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