Maryland state law requires that children be immunized against measles, polio, rubella (German measles), diphtheria, pertussis (whooping cough) and tetanus before they can enter any preschool or elementary school.

It applies to students transferring to Maryland schools from other jurisdiction as well as children who are attending school for the first time.

In addition, an emergency state regulation which is in effect through Sept. 20 requires that all students in the seventh through 12th grades must show proof of measles immunity before they can start school.

The principal can be fined if convicted of knowingly admitting any child who does not have proof that the series has been completed. Some exceptions are allowed, such as in the case of religious convictions when there is no epidemic, or if parents show that their child is in the process of complying with the regulations.

Many children are not immunized until they approach school age, according to the Center for Disease Control in Atlanta. This leaves them susceptible to disease at a time when they are most vulnerable - most of the diseases are likely to strike young children and the consequences are often most severe in this age group.

The Center of Disease Control recommends that children be vaccinated at the following times:

Polio - The oral polio vaccine should begin when a child is about 2 months old. Three doses are given during the first six months to one year. A booster dose is sometimes given at 15 to 18 months of age. Another dose is recommended at school entry.

Measles, Mumps, Rubella - Each of these disease requires only one immunization. Some combination vaccines are available that make it possible for one injection to provide immunization for all three diseases. The immunization is recommended at approximately 15 months of age.

Diphtheria-Tetanus-Pertussis - A combination vaccine is given in four doses, beginning when the child is about 2 months old. A booster dose is given at school entry.

The following Montgomery and Prince George's County health centers offer free immunizations against basic preventable childhood diseases. It is advisable to call first.

Montgomery County

Bethesda Health Center, 4848 Cordell Ave., Bethesda, 2 to 4 p.m. Tuesdays. 654-5525.

Rocking Horse Health Center, 4701 Randolph Rd., Rockville, 2 to 4 p.m. Mondays. 881-9240.

Silver Spring Health Center, immunizations at Piney Branch Middle School, 7510 Maple Ave., Takoma Park, 8 to 9 a.m. weekdays by appointment only. 270-6400.

Northeast Health Center, 3460 Olney Laytonsville Rd., Olney, 2 to 4 p.m. Mondays. 774-0880.

Gaithersburg Health Center, 542 N, Frederick Ave., Gaithersburg, 2 to 4 p.m. Tuesdays. 926-6000.

Central Services Clinic, 12701 Twinbrook Parkway, Rockville, 5 to 8 p.m. Mondays. 279-1650.

Prince George's County

Ager Road Clinic, 6111 Ager Rd., Hyattsville. 422-3132.

Baden Health Clinic, Baden Westwood Rd., Baden. 88-1197.

John Carroll Clinic, 1400 Nalley Terrace, landover. 773-0711.

O.W. Phair Clinic, West St., Laurel. 725-8546.

Paint Branch Clinic, 5101 Pierce Ave., College Park. 345-2050.

Fairmont Heights Clinic, 6100 Jost St., Fairmont Heights. 925-885.