1st Regiment, Pennsylvania Infantry (3 months, 1861)
This regiment was organized at Harrisburg April 20, 1861. It mustered out July 27, 1861. 
Companies in this Regiment with the Counties of Origin
Men oftenThe First Regiment was recruited in:
Company A - many men from Bethlehem, Northampton County
Company B - many men from Easton, Northampton County
Company C - many men from Easton, Northampton County
Company D - many men from Easton, Northampton County
Company E - many men from Harrisburg, Dauphin County
Company F - many men from Lancaster, Lancaster County
Company G - many men from Reading, Berks County
Company H - many men from Easton, Northampton County
Company I - many men from Allentown, Lehigh County
Company K - many men from Lancaster, Lancaster County
enlisted in a company recruited in the counties where they lived though not always. After many battles, companies might be combined because so many men were killed or wounded. However if you are unsure which company your ancestor was in, try the company recruited in his county first.
National Park Service, The Civil War Soldiers and Sailors System, is searchable by soldier's name and state. It contains basic facts about soldiers on both sides of the Civil War, a list of regiments, descriptions of significant battles, sources of the information, and suggestions for where to find additional information.