Banks are showing interest in the credit card market again. Last month, Commonwealth Bank cut purchase rates on its cards, and last week Westpac launched two new black cards.For a few years following the financial crisis, banks took a cautious approach to managing their credit card portfolios. But strong retail asset quality over the past year has encouraged some lenders to grow their credit card portfolios.The pay-off for banks is that credit cards, which have interest rates as high as 20 per cent, offer high margins.Commonwealth Bank's chief financial officer, David Craig, told analysts at last month's interim results briefing: "We have continued our push into high profit products such as credit cards and personal loans. We're growing significantly above system in this space and that's a level of 13 per cent growth."Craig said the move into credit cards and personal loans was one of the main factors in the bank's increase in margin during the December half.In February, Commonwealth cut the purchase rate and the cash advance rate on a number of its credit cards by 25 basis points.Westpac is promoting two new cards at the premium end - Altitude Black and Earth Black. The bank is offering companion MasterCard and American Express cards, with up to three points for each dollar spent. Westpac's black cards come with a concierge service, a complimentary pass to VIP airport lounges, a travel agency service, preferred status at Starwood hotels and complimentary insurance cover. The annual fee is A$395.According to Australian Prudential Regulation Authority figures published on Friday, total credit card balances fell by 2.1 per cent over the 12 months to February. Commonwealth Bank, Bendigo and Adelaide Bank, and HSBC were the only three card issuers to record increases in their card balances over the 12 months.