The University of Maryland is still looking for some itty bitty volunteers to take part in its swine flu vaccine trial: babies 6 to 35 months.

Investigators have had no problem recruiting adults and older children in the trails, which started in adults earlier this month and in children last week. In fact, they had so many 18-64 year olds try to volunteer, they had to use a lottery to make the final cut. Even older children, many of whom were signed up by their doctor parents, have been an easy find.

But the smallest of all test subjects have proved a challenge for researchers. Officials say it isn't because parents are reluctant to test the experimental inoculation on their wee ones. They've had great response from all age groups. Rather, it seems babies' recommended -- and rigorous -- vaccine schedule might be getting in the way. From 12 to 18 months alone, children can receive various vaccines from shots against measles, mumps and rubella to hepatitus A. But the swine flu study requires that babies have not had recent inoculations and will not be vaccinated soon after they receive the H1N1 shot.

If you are interested in volunteering your tot and want to know if your child qualifies, call the University of Maryland's Center for Vaccine Development for details: 410-706-6156. Vaccinations are taking place now at the University of Maryland's Ambulatory Pediatric Center in Baltimore and at clinics in Annapolis and Frederick. The university, one of a handful of vaccination sites across the nation, hopes to test 40 children in the 6 to 35 month old range.