Ad Infuse on Wednesday became the latest mobile advertising upstart to tap into growing investor interest in the slowly emerging mobile ad market. The San Francisco-based startup said it got $12 million from SoftBank Capital, ComVentures, and Storm Ventures.

The investment brings the two-year-old company's funding to $18 million

Ad Infuse, which works with mobile operators, content publishers, and advertisers to distribute personalized ads to consumers' mobile devices, said it manages mobile ad campaigns for firms such as Unilever, Procter & Gamble, EA Mobile, BMW, Microsoft, and Dell.

Ad Infuse is one of about a dozen venture-backed startups that are targeting the mobile market. Others include AdMob, which has received roughly $18 million in funding; Millennial Media, which took $6.3 million last year; and Quattro, which got $12.3 million in funding in September.

Despite the interest, the mobile ad market has started  slowly because of an array of problems that range from technical and bandwidth issues to potential consumer rejection of the whole concept as an invasion of privacy.

Still, research firm eMarketer expects worldwide spending on mobile ads to grow from $1.5 billion in 2006 to $13.8 billion in 2011. In the U.S. the firm expects spending on mobile ads to grow from $421 million in 2006 to $4.8 billion in 2011.

"We have found that the response rates and the effectiveness of advertising on mobile phones to be very limited because of bandwidth issues," said Craig Macdonald, vice president of marketing, products and alliances for Covario, a San Diego-based interactive media advertising specialist.

The potential for a personal ad medium like the mobile phone is very attractive to marketers, he said, but the slow rate at which web pages render on phones has a dampening effect on consumer participation.

"It is mostly a technological issue, but we think that the technology will catch-up with the desires of marketers and end users in the next couple of years," Mr. Macdonald said.